There are several ways to add courses in Aurora Student. Check for instructions under Steps to Registration.

When adding courses Aurora Student will check such things as: if you are eligible for the course or section; that you have the prerequisite for the course; and that your timetable will be free of any time conflicts.
 
In some cases, such as if you are repeating a course, you will first need to obtain approval and have a course override entered before being permitted to add a course.

There are also circumstances in which courses may not be added by students through self service and instead can only be added by your faculty or school office.