Only certain courses, such as project courses, industry placements or internships, require approval by the institute/school before you can be enrolled. You will see this clearly indicated in the Enrolment Information field in mySC and in the error message that appears when you try to enrol.
In this situation you will need to contact your institute/school and speak to the particular course coordinator to get permission to enrol. If you are approved, the institute/school will advise Student HQ to have you enrolled into the course. When the enrolment is completed you will receive a confirmation email and be able to view the relevant course in mySC.