Hello Laures
As stated in the Helpful Tips PDF, the proper way to Drop a course assigned by error is:
If you want to drop a student from a course which was mistakenly added, you can
remove it by selecting a drop date prior to the enrollment date. (See the Dropped date
in the Scheduling > Student Schedule program.)
Unless you do that, and confirm you want to also delete grades and attendance, there may be references to this course period in the database.
Also, another thing to check is whether the Teacher has created assignments for this course period, and if he has taken eligibility for this course period (eligibility cannot be deleted from the interface).
If after all those checks, you still don't have the Delete button reappearing, the only option left is to edit and reuse this course period.
Please let us know