Not able to navigate screens in site

Sites don't inherently have a "task list" wherein a user would see an assigned task that they would need to accept (like when the new task is assigned to a group or a small list of users, etc).  Is this what you want?  Or are you expecting that the user would see the subsequent task immediately, as mentioned already by Danny above?

If your desire is for users to see a "task list" that they can accept an assigned task (i.e. from an action previously kicked off by a different user in the same site), you will need to build a separate form in the site where a user's currently assigned tasks are queried via a!queryProcessAnalytics, and then you can give them clickable task links to open their task and accept it.

thank you for your reply. I will try to implement this thing for accepting the tasks by users from site. these clickable task links which you're suggesting, is it the same as we get in email as view task once that task is assigned to me? Can I configure url for their task? 

Yeah you can configure links within the grid to bring you directly to the associated task. I'd suggest looking at the Task Report Tutorial, which provides a great walkthrough for how to configure several aspects of task reports.

Yeah you can configure links within the grid to bring you directly to the associated task. I'd suggest looking at the Task Report Tutorial, which provides a great walkthrough for how to configure several aspects of task reports.

code for predefined filter in task report tutorial link is throwing error. Could not display interface. Please check definition and inputs. Interface Definition: Expression evaluation error at function a!dropdownField [line 19]: A dropdown component [label=“Status”] has an invalid value for “value”. Value cannot be null.

Below is the code 

a!localVariables(
 local!statusFilter: null,
  local!taskStatuses: {
    "Assigned",
    "Accepted",
    "Completed",
    "Not Started",
    "Cancelled",
    "Paused",
    "Unattended",
    "Aborted",
    "Cancelled By Exception",
    "Submitted",
    "Running",
    "Error"
  },
 a!sectionLayout(
   contents:{
     a!dropdownField(
       label: "Status",
       placeholder: "Pick a status",
       choiceLabels: { "Assigned", "Accepted", "Completed", "Not Started" },
       choiceValues: enumerate(4),
       value: local!statusFilter,
       saveInto: local!statusFilter
     ),
      a!gridField(
        label: "My Tasks",
        instructions: "A list of all tasks for the current user",
        labelPosition: "ABOVE",
        data: a!queryProcessAnalytics(
          report: cons!MY_TASKS_REPORT,
          query: a!query(
            pagingInfo: fv!pagingInfo,
           filter: a!queryFilter(
               field: "c5",
               operator: "=",
               value: local!statusFilter,
               applyWhen: not(
                  isnull(
                    local!statusFilter
                  )
                ),
             ),
          ),
        ),
        columns: {
          a!gridColumn(
            label: "Name",
            sortField: "c0",
            value: a!linkField(
              links: a!processTaskLink(
                label: fv!row.c0,
                task: fv!identifier
              )
            )
          ),
          a!gridColumn(
            label: "Process",
            sortField: "c3",
            value: fv!row.c3
          ),
          a!gridColumn(
            label: "Status",
            sortField: "c5",
            value: a!forEach(
              items: fv!row.c5,
              expression: if(
                not(isnull(fv!row.c5)),
                fn!index(
                  local!taskStatuses,tointeger(fv!item)+1,"Other"
                ),
                tostring(fv!item)
              )
            )
          )
        },
        rowHeader: 1
      )
   }
 )
)

Try this 

a!localVariables(
  local!statusFilter:null,
  local!taskStatuses: {
    "Assigned",
    "Accepted",
    "Completed",
    "Not Started",
    "Cancelled",
    "Paused",
    "Unattended",
    "Aborted",
    "Cancelled By Exception",
    "Submitted",
    "Running",
    "Error"
  },
  a!sectionLayout(
    contents: {
      a!dropdownField(
        label: "Status",
        placeholderLabel: "Pick a status",
        choiceLabels: {
          "Assigned",
          "Accepted",
          "Completed",
          "Not Started"
        },
        choiceValues: enumerate(
          4
        ),
        value: local!statusFilter,
        saveInto: local!statusFilter
      ),
      a!gridField(
        label: "My Tasks",
        instructions: "A list of all tasks for the current user",
        labelPosition: "ABOVE",
        data: a!queryProcessAnalytics(
          report: cons!MY_TASKS_REPORT,
          query: a!query(
            pagingInfo: fv!pagingInfo,
            filter: a!queryFilter(
              field: "c5",
              operator: "=",
              value: local!statusFilter,
              applyWhen: not(
                isnull(
                  local!statusFilter
                )
              ),
              
            ),
            
          ),
          
        ),
        columns: {
          a!gridColumn(
            label: "Name",
            sortField: "c0",
            value: a!linkField(
              links: a!processTaskLink(
                label: fv!row.c0,
                task: fv!identifier
              )
            )
          ),
          a!gridColumn(
            label: "Process",
            sortField: "c3",
            value: fv!row.c3
          ),
          a!gridColumn(
            label: "Status",
            sortField: "c5",
            value: a!forEach(
              items: fv!row.c5,
              expression: if(
                not(
                  isnull(
                    fv!row.c5
                  )
                ),
                fn!index(
                  local!taskStatuses,
                  tointeger(
                    fv!item
                  ) + 1,
                  "Other"
                ),
                tostring(
                  fv!item
                )
              )
            )
          )
        },
        rowHeader: 1
      )
    }
  )
)

thanks it worked :)