error in function

is it because of not using index() function 

is it because of not using index() function 

Honestly the small snippet you've posted in your screenshot doesn't tell us much.  What type of field is it?  What are you trying to accomplish?  Can you post a larger code snippet, preferably in a Code Box?

/*Row layout for each row of the Person grid to add new Person.*/
=load(
  local!tmods_user: if(
      or(
        rule!SCE_Case_isEmpty(ri!items),
        rule!SCE_Case_isEmpty(ri!items[ri!index].id)
      ),
      if(
        rule!SCE_Case_isEmpty(ri!items[ri!index].emp_num),
        null,
        rule!TMODS_getUserByTeamMemberId(ri!items[ri!index].emp_num)
      ),
      rule!TMODS_getUsers(
        value: ri!items[ri!index].id,
        batchSize:1
      )
  ),
  local!identified: if(ri!items[ri!index].type=cons!SCE_CASE_EMP_TYPES[1], true, false),
  a!gridRowLayout(
  /*Index of the row*/
  id: ri!index,
  contents: {
    a!dropdownField_20r2(
      label: rule!SCE_Case_translateKey(
        "translate.label.type"
      ) & ri!index,
      choiceLabels: a!forEach(
        cons!SCE_CASE_EMP_TYPES,
        rule!SCE_Case_convertEnToFr(
          fv!item
        )
      ),
      choiceValues: cons!SCE_CASE_EMP_TYPES,
      placeholderLabel: rule!SCE_Case_translateKey(
        cons!SCE_CASE_PLACEHOLDER_VALUE
      ),
      value: ri!items[ri!index].type,
      saveInto: {
        /*This is where we destroy the row of data when the user flips the type */
        a!save(ri!items[ri!index],null),
        /* This is where we save the type */
        ri!items[ri!index].type,
        if(
          ri!items[ri!index].type = cons!SCE_CASE_EMP_TYPES[1],
          {
            /*when identified*/
            a!save(
              ri!items[ri!index].affiliation,
              cons!SCE_CASE_AFFILIATION[1]
            ),
            a!save(
              local!identified,
              true()
            )
          },
          {
        
            a!save(
              ri!items[ri!index].affiliation,
              cons!SCE_CASE_AFFILIATION[2]
            ),
            a!save(
              local!identified,
              false()
            )
          }
        )
      },
      required: true
    ),
    if(
      local!identified,
      a!textField(
        label: rule!SCE_Case_translateKey(
          "translate.label.affiliation"
        ) & ri!index,
        value: ri!items[ri!index].affiliation,
        saveInto: ri!items[ri!index].affiliation,
        readOnly: true
      ),
      a!dropdownField_20r2(
        label: rule!SCE_Case_translateKey(
          "translate.label.affiliation"
        ) & ri!index,
        choiceLabels: a!forEach(
          cons!SCE_CASE_AFFILIATION,
          rule!SCE_Case_translateKey(
            fv!item
          )
        ),
        choiceValues: cons!SCE_CASE_AFFILIATION,
        placeholderLabel: rule!SCE_Case_translateKey(
          cons!SCE_CASE_PLACEHOLDER_VALUE
        ),
        value: ri!items[ri!index].affiliation,
        saveInto: {
          ri!items[ri!index].affiliation,
          a!save(
            ri!items[ri!index].last_updated_by,
            loggedInUser()
          )
          ,
          a!save(
            ri!items[ri!index].last_updated_on,
            now()
          )
        },
        required: true
      )
    ),
    if(
      local!identified,
      a!pickerFieldCustom(
        label: rule!SCE_Case_translateKey(
          "translate.label.search"
        ) & ri!index,
        maxSelections: 1,
        suggestFunction: rule!TMODS_customPicker(
          filter: _
        ),
        selectedLabels: if(
            rule!SCE_Case_isEmpty(
              local!tmods_user
            ),
            null,
            local!tmods_user.preferred_given_name & " " & local!tmods_user.family_name & " (" & local!tmods_user.enterprise_login_id & ") "
          ),
        value: if(
          rule!SCE_Case_isEmpty(
            local!tmods_user
          ),
          null,
          local!tmods_user.preferred_given_name & " " & local!tmods_user.family_name & " (" & local!tmods_user.enterprise_login_id & ") "
        ),
        saveInto: {
          local!tmods_user,
          a!save(
            ri!items[ri!index].affiliation,
            if(
              rule!SCE_Case_isEmpty(local!tmods_user),
              cons!SCE_CASE_AFFILIATION[1],
              if(
                or(/*retired or terminated employees*/
                  local!tmods_user.employment_status_cd="RET", 
                  local!tmods_user.employment_status_cd="TERM"
                ),
                cons!SCE_CASE_AFFILIATION[3],
                cons!SCE_CASE_AFFILIATION[1]
              )
            )
          ),
          a!save(
            ri!items[ri!index].first_name,
            if(
              rule!SCE_Case_isEmpty(local!tmods_user),
              null,
              local!tmods_user.given_name
            )
          ),
          a!save(
            ri!items[ri!index].last_name,
            if(
              rule!SCE_Case_isEmpty(local!tmods_user),
              null,
              local!tmods_user.family_name
            )
          ),
          a!save(
            ri!items[ri!index].pref_name,
            if(
              rule!SCE_Case_isEmpty(local!tmods_user),
              null,
              local!tmods_user.preferred_given_name
            )
          ),
          a!save(
            ri!items[ri!index].job_title,
            if(
              rule!SCE_Case_isEmpty(local!tmods_user),
              null,
              local!tmods_user.position_title
            )
          ),          
          a!save(
            ri!items[ri!index].phone,
            if(
              rule!SCE_Case_isEmpty(local!tmods_user),
              null,
              local!tmods_user.work_phone_no
            )
          ),     
          a!save(
            ri!items[ri!index].email,
            if(
              rule!SCE_Case_isEmpty(local!tmods_user),
              null,
              local!tmods_user.work_email_address_text
            )
          ),   
          a!save(
            ri!items[ri!index].telus_id,
            if(
              rule!SCE_Case_isEmpty(local!tmods_user),
              null,
              local!tmods_user.enterprise_login_id
            )
          ),
          a!save(
            ri!items[ri!index].emp_num,
            if(
              rule!SCE_Case_isEmpty(local!tmods_user),
              null,
              local!tmods_user.team_member_id
            )
          ),
          a!save(
            ri!items[ri!index].person_type,
            if(
              rule!SCE_Case_isEmpty(local!tmods_user),
              null,
              ri!items[ri!index].person_type
            )
          ),
          if(
            or(
              rule!SCE_Case_isBlank(ri!items[ri!index]),
              rule!SCE_Case_isBlank(ri!items[ri!index].person_id)
            ),
            {
              /* add a new TMOS person */
              a!save(
                ri!items[ri!index].person_id,
                null
              ),
              a!save(
                ri!items[ri!index].added_by,
                loggedInUser()
              ),
              a!save(
                ri!items[ri!index].added_on,
                now()
              )
            },
            {
            /*update an existing TMODS person - DO NOTHERE here, so won't duplicate the person*/
            }
          ),
          a!save(
            ri!items[ri!index].last_updated_by,
            loggedInUser()
          ),
          a!save(
            ri!items[ri!index].last_updated_on,
            now()
          ),
          a!save(
            ri!items[ri!index].is_active,
            true()
          ),
          a!save(
            ri!items[ri!index].type,
            cons!SCE_CASE_EMP_TYPES[1]
          ),
          a!save(
            local!identified,
            true()
          )
        },
        required: true
      ),
      a!pickerFieldCustom(
        label: rule!SCE_Case_translateKey(
          "translate.label.preferredName"
        ) & ri!index,
        placeholder: "Type to select the Person's details",
        maxSelections: 1,
        suggestFunction: rule!SCE_Case_personCustomPicker(
          filter: _
        ),
        selectedLabels: if(
          or(
            rule!SCE_Case_isEmpty(ri!items),
            rule!SCE_Case_isBlank(ri!items[ri!index].last_name)
          ),
          null,
          ri!items[ri!index].pref_name & " " & ri!items[ri!index].last_name
        ),
        value: if(
          or(
            rule!SCE_Case_isEmpty(ri!items),
            rule!SCE_Case_isBlank(ri!items[ri!index].last_name)
          ),
          null,
          ri!items[ri!index].pref_name & " " & ri!items[ri!index].last_name
        ),
        saveInto: {
          ri!items[ri!index],
          if(
            or(
              /* person not found i.e. new person */
              rule!SCE_Case_isEmpty(ri!items[ri!index]),
              rule!SCE_Case_isBlank(ri!items[ri!index].person_id),
              /* person found but not in this case */
              ri!items[ri!index].case_id <> ri!caseId
            ),
            {
              /*clear person picker or add a new person*/
              a!save(
                ri!items[ri!index].person_id,
                null
              ),
              a!save(
                ri!items[ri!index].added_by,
                loggedInUser()
              ),
              a!save(
                ri!items[ri!index].added_on,
                now()
              )
            },
            {}
          ),
          a!save(
            ri!items[ri!index].last_updated_by,
            loggedInUser()
          ),
          a!save(
            ri!items[ri!index].last_updated_on,
            now()
          ),
          a!save(
            ri!items[ri!index].is_active,
            true()
          ),
          a!save(
            ri!items[ri!index].type,
            cons!SCE_CASE_EMP_TYPES[2]
          ),
          a!save(
            local!identified,
            false()
          )
        },
        required: true
      )
    ),
    a!textField(
      label: rule!SCE_Case_translateKey(
        "translate.label.userID"
      ) & ri!index,
      value: ri!items[ri!index].telus_id,
      saveInto: {
        ri!items[ri!index].telus_id,
        a!save(
          ri!items[ri!index].last_updated_by,
          loggedInUser()
        ),
        a!save(
          ri!items[ri!index].last_updated_on,
          now()
        )
      },
      readOnly: true    
    ),
    a!textField(
      label: rule!SCE_Case_translateKey(
        "translate.label.firstName"
      ) & ri!index,
      value: ri!items[ri!index].first_name,
      saveInto: {
        ri!items[ri!index].first_name,
        a!save(
          ri!items[ri!index].is_active,
          true()
        ),
        a!save(
          ri!items[ri!index].last_updated_by,
          loggedInUser()
        ),
        a!save(
          ri!items[ri!index].last_updated_on,
          now()
        )
      },
      readOnly:local!identified,
      required: not(local!identified)
    ),
    a!textField(
      label: rule!SCE_Case_translateKey(
        "translate.label.lastName"
      ) & ri!index,
      value: ri!items[ri!index].last_name,
      saveInto: {
        ri!items[ri!index].last_name,
        a!save(
          ri!items[ri!index].last_updated_by,
          loggedInUser()
        ),
        a!save(
          ri!items[ri!index].last_updated_on,
          now()
        )
      },
      readonly: local!identified,
      required: not(local!identified)
    ),  
    a!textField(
      label: rule!SCE_Case_translateKey(
        "translate.label.preferredName"
      ) & ri!index,
      value: ri!items[ri!index].pref_name,
      saveInto: {
        ri!items[ri!index].pref_name,
        a!save(
          ri!items[ri!index].last_updated_by,
          loggedInUser()
        ),
        a!save(
          ri!items[ri!index].last_updated_on,
          now()
        )
      },
      readonly: local!identified
    ),    
    a!textField(
      label: rule!SCE_Case_translateKey(
        "translate.label.jobTitle"
      ) & ri!index,
      value: ri!items[ri!index].job_title,
      saveInto: {
        ri!items[ri!index].job_title,
        a!save(
          ri!items[ri!index].last_updated_by,
          loggedInUser()
        ),
        a!save(
          ri!items[ri!index].last_updated_on,
          now()
        )
      },
      readonly:local!identified
    ),
    a!textField(
      label: rule!SCE_Case_translateKey(
        "translate.label.phoneNumber"
      ) & ri!index,
      value: ri!items[ri!index].phone,
      saveInto: {
        ri!items[ri!index].phone,
        a!save(
          ri!items[ri!index].last_updated_by,
          loggedInUser()
        ),
        a!save(
          ri!items[ri!index].last_updated_on,
          now()
        )
      },
      readonly:local!identified
    ),
    a!textField(
      label: rule!SCE_Case_translateKey(
        "choiceList.attachType.email"
      ) & ri!index,
      value: ri!items[ri!index].email,
      saveInto: {
        ri!items[ri!index].email,
        a!save(
          ri!items[ri!index].last_updated_by,
          loggedInUser()
        ),
        a!save(
          ri!items[ri!index].last_updated_on,
          now()
        )
      },
      readonly:local!identified,
      validations: rule!SCE_Case_validateEmail(
        ri!items[ri!index].email
      )
    ),
    a!textField(
      label: rule!SCE_Case_translateKey(
        "translate.label.company"
      ) & ri!index,
      value: ri!items[ri!index].company,
      saveInto: {
        ri!items[ri!index].company,
        a!save(
          ri!items[ri!index].last_updated_by,
          loggedInUser()
        ),
        a!save(
          ri!items[ri!index].last_updated_on,
          now()
        )
      },
      readonly: local!identified
    ),
    a!dropdownField_20r2(
      label: rule!SCE_Case_translateKey(
        "translate.label.personType"
      ) & ri!index,
      choiceLabels: a!forEach(
        cons!SCE_CASE_PERSON_TYPES,
        rule!SCE_Case_convertEnToFr(
          fv!item
        )
      ),
      choiceValues: cons!SCE_CASE_PERSON_TYPES,
      placeholderLabel: rule!SCE_Case_translateKey(
        cons!SCE_CASE_PLACEHOLDER_VALUE
      ),
      value: ri!items[ri!index].person_type,
      saveInto: {
        ri!items[ri!index].person_type,
        a!save(
          ri!items[ri!index].last_updated_by,
          loggedInUser()
        ),
        a!save(
          ri!items[ri!index].last_updated_on,
          now()
        )
      },
      required: true
    ),
    a!imageField(
      label: rule!SCE_Case_translateKey(
        "translate.value.delete"
      ) & ri!index,
      images: a!documentImage(
        document: a!iconIndicator(
          "REMOVE"
        ),
        altText: rule!SCE_Case_translateKey(
          "translate.value.remove"
        ),
        link: a!dynamicLink(
          value: ri!index,
          saveInto: {
            if(
              rule!SCE_Case_isBlank(
                ri!items[ri!index].person_id
              ),
              {},
              {
                a!save(
                  ri!items[ri!index].is_active,
                  false()
                ),
                a!save(
                  ri!items[ri!index].last_updated_by,
                  loggedInUser()
                ),
                a!save(
                  ri!items[ri!index].last_updated_on,
                  now()
                ),
                a!save(
                  ri!deletedPersonDetails,
                  append(
                    ri!deletedPersonDetails,
                    ri!items[ri!index]
                  )
                )
              }
            ),
            a!save(
              ri!items,
              remove(
                ri!items,
                save!value
              )
            ),
            /* When modifying the size of the array used in a!applyComponents,  */
            /* make the same change in the "token" array variable               */
            a!save(
              ri!itemsToken,
              remove(
                ri!itemsToken,
                save!value
              )
            )
          }
        )
      ),
      size: "ICON"
    )
  }
)
)

This code give me error:

May be not used index. 

From the error message, I would assume that ri!items being passed to SCE_Case_isEmpty is null.

how it is resolved

If ri!items is empty, then any use of square brackets i.e. "ri!items[ri!index]" will cause this issue.  The easiest thing here would be to ensure ri!items is not empty.  If you're using the interface designer, provide it some value after clicking the "test" button and see whether the errors go away.

Who originally designed this interface?  It's doing some pretty complicated things and it is NOT designed to handle an empty "ri!items" value without breaking.  Can you consult whoever originally designed it and seek to gain a better understanding of its functionality from them?  It's really difficult for us here to explain all the little details, even when we can see things that are wrong.