Most courses in science disciplines require that you know some foundational material, i.e. that you have taken some previous course(s). These are called Prerequisites. Generally the course enrolment system does not enforce prerequisites. You may be able to enrol without having taken the prerequisites; but if you do you are likely to be removed from the course by the department that offers it.
Naturally we want students to succeed and prerequisites are one way of advising students about the order in which they should take courses.
If you perform poorly in a course that you took without having previously taken the prerequisite, the lack of a prerequisite, even if it was approved by the instructor, is not viewed as valid reason for any subsequent consideration (e.g. grounds for dropping a course after the drop deadline).
Course Credit Exclusions
Some courses cover similar material. Naturally a student should not receive credit for taking both courses if they are quite similar. Such courses are identified in a list labelled Course Credit Exclusion that is part of the course description.
The course enrolment system does not prevent you from enrolling in a course that is an exclusion with one you have already taken. If you do your transcript will show that the first course is annotated with NCR meaning “no credit retained”.