This error indicates that one of the students being withdrawn is missing the Student role.  For each student being withdrawn ensure the Student role is marked on their enrollment record for the year.
  1. Go to Core > Users/Access > Profile > Edit User Profile Data
  2. Find and open the student being withdrawn
  3. Under System Information click Student Enrollment
  4. Next to the School Year being promoted click Edit
  5. Ensure Student is marked under Selected Roles