Take Up Code isn’t really a place where you enroll. Not in the normal meaning of enroll, anyway. You can register for classes that repeat so you can attend during an upcoming class session event.
Enrollment in a normal school implies that you’re a student for a certain amount of time and then you become an alumni and stop taking classes. It’s different here. When you register for a class, you can take that class again and again as many times as you want or need whenever it’s scheduled.
Some classes build on others. Especially the Essential 42 set of classes. For these classes, the best time to “enroll” is before the next cycle begins. That lets you take the sessions in order so you won’t get lost or confused because you missed a session.