Try enrolling the students in smaller groups to see if you can locate a single student that caused the error.

Once the student has been located check to see if the student was previously enrolled.

1. Go to Core > Users > Edit User Profile Data
2. Search for the student.
3. Scroll down to Student Enrollment
4. If the student had a previous enrollment and was withdrawn select the option to Re-Enroll the student.
5. Determine whether or not you would like to include future progressions.
6. Choose the year that you would like to re-enroll the student into.
7. Save and exit.