Tara,
This usually happens because of one of the following areas:
1. Participant Assignments still tied to the group.
2. Staffing Assignments still tied to the group.
3. Attendance still tied to the group.
4. Forms or Form Submissions still tied to the group.
5. Volunteer Forms tied to the group.
If any of the above exist, you will not be able to delete the Breakout Group. My best guess without knowing the details is that this is most likely related to Attendance or Staffing assignments (possibly even inactive assignments).
In any case, you may want to update the ticket now to indicate what you'd like to happen to any of the above tied to the Breakout Group - delete the entry, or just remove the Breakout Group. Without knowing for sure what you intend for the attached items, we can't issue the appropriate commands behind the scenes to remove the groups.
In Him,
-Peter Schott