This is often caused by trying to enroll a Candidate that already has enrollment rows in their Core profile.

As a Platform Manager:
  1. Navigate to Core.
  2. Go to People Finder.
  3. Search/select your user.
  4. Navigate to the Access tab.
  5. To the left of their user role, click the Edit pencil icon.
  6. Select Edit School Enrollment.
  7. Click the Delete icon to the right of any of the listed enrollment rows.
  8. Save.
  9. Go back to the enroll task, and enroll the candidate.