Take Up Code isn’t really a place where you enroll. Not in the normal meaning of enroll, anyway. You can register for individual classes or register for a bundle of classes.
Enrollment 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.
Some classes build on others. Especially the Essential 42 set of classes which repeat every 2 months. For these classes, the best time to “enroll” is before the next cycle begins. That lets you take the classes in order so you won’t get lost or confused because you missed a class.