Why can't I un-enroll from a course?

This is likely because you've automatically been enrolled in that course through a track enrollment. If you view your "My Classroom" page you can see the tracks that you are currently enrolled in. If the course appears in that track it means you will automatically be enrolled in that course until you un-enroll from the track. 

While you are enrolled in a related track, the option to individually enroll or un-enroll from any course in that track is disabled.