What it means

The department offering the class has limited the number of students who can be on this class’s wait list. That list is now full.

While it's possible to see an indication of available seats, that number is related to the department's enrollment management. Once the class availability is listed as "Closed", you cannot enroll in the class or the wait list.

cancel Enrollment failed:
You were not enrolled. The course section you selected is full and there is not room on the wait list.

What to do

Since you can’t add yourself to the wait list now, check back from time to time to see if spots on the wait list open up. See: Course Search & Enroll - Wait Lists

Or, contact the department offering the class, as they manage the wait list. Although departments don’t generally share information like how long the wait list is, some may adjust the number of available seats (or wait list size) in high-demand classes. See: Course Search & Enroll - Permission and Approval for Course Enrollment