I am a non PRO subscriber. When I try to get a new free course, it says that I am already studying the maximum

The free course limit is the number of free and private courses altogether.  The course slot will be available:

1.  If you archive one of the free courses.

2.  If you delete one of your private/custom courses. 

Alternatively, you can exchange 100 rubies in the store for an additional free course slot.

Note: Archiving a private course doesn't release the slot, you'll need to delete it.