If you were enrolled in a D2L workshop in a previous semester, you were unenrolled at the end of that semester. If you need to enroll in the workshop again, D2L will likely say that you are still enrolled in the course, but when you click Open Course, you get this message:
Fortunately, there is an easy fix!
In order to re-enroll in the course, you will need to manually un-enroll yourself first. Click the ellipses next to the Open Course button and then select Unenroll from the dropdown.
Then, click Enroll in Course. You should now be able to access the course again.