validation

Check with below code, If it throws same error check the groups security.

if(
  not(isusernametaken(ri!approver_txt)),
  cons!CR_NEXT_AUTHORITY_VALIDATION_MESSAGE[2],
  if(
    ri!approver_txt = loggedInUser(),
    cons!CR_NEXT_AUTHORITY_VALIDATION_MESSAGE[1],
    if(
      a!isUserMemberOfGroup(
        ri!approver_txt,
        {
          cons!CR_GRP_ALL_CCC_USER_GROUP,
          cons!CR_GRP_BOARD_OF_DIRECTORS_GROUP
        }
      ),
      cons!CR_NEXT_AUTHORITY_VALIDATION_MESSAGE[3],
      null
    )
  )
)

this logic will give this error

 if(
        isusermemberofgroup(
          ri!approver_txt,
          {
            cons!CR_GRP_BOARD_OF_DIRECTORS_GROUP
          }
        ),
        cons!CR_NEXT_AUTHORITY_VALIDATION_MESSAGE[3],
        if(
          isusermemberofgroup(
            ri!approver_txt,
            {
              cons!CR_APP_GRP_HOC_GROUP,
              
            }
          ),
          cons!CR_NEXT_AUTHORITY_VALIDATION_MESSAGE[3],
          if(
            isusermemberofgroup(
              ri!approver_txt,
              {
                cons!CR_APP_GRP_DCEO_GROUP
              }
            ),
            cons!CR_NEXT_AUTHORITY_VALIDATION_MESSAGE[3],
            if(
              isusermemberofgroup(
                ri!approver_txt,
                {
                  cons!CR_APP_GRP_CEO_GROUP
                }
              ),
              cons!CR_NEXT_AUTHORITY_VALIDATION_MESSAGE[3],
              null
            )
          )
        )
      )

this worked

Please note also that if you're on a reasonably modern version of Appian, you should be using the newer function a!isUserMemberOfGroups(), which automatically handles checking many groups (and built-in logic for requiring the user to be a member of all listed groups or any listed groups).