This usually means the User has 2 Applications submitted and cannot change the Checklist until one Application is removed.

To correct this navigate to:
  1. Enrollment Management > People Finder > Search the Candidate
  2. Click on the Checklist Tab
  3. Click the Red x next to the Application that is no longer needed
  4. Click Change Checklist and Select the New Checklist from the dropdown
  5. Click Save
If the issue is not resolved by the above, then it could also be How do I change the checklist assigned to a candidate?