When the enrollment in a course I teach is listed "closed" to new enrollees, the action has been controlled by a computer not me, based upon enrollment limitations and when you tried to enroll.  Once the course closes, you may gain admission only if someone else drops the course. There are often “drop and adds” during the period leading up to the start of each semester. Gaining access is  “first come first served.” Check often online to see if someone has dropped the course; if so, you can register then. In  future semesters, remember always to enroll at the first opportunity.