Check if you have a hold on you account.
From CSUN: Holds are “indicators” on your student record that require your attention and action. There are many kinds of holds, from “advisement” to “undecided major.” For example, an Advisement Hold means that you must meet with your academic advisor before enrolling in classes. Some holds, called Registration Holds, block enrollment in classes.
Click link below to learn how to check if you have a hold on your account. http://goo.gl/vtLBmk