This document explains the meaning of the "You do not have access to perform this transaction at this time" error message and how you can resolve it.
You are trying to enroll in a class after the enrollment deadline.
If you are seeing this error message during enrollment time, it's possible you're trying to enroll in the wrong term.
Make sure you are trying to enroll in classes in the correct term.
In your cart, change the term from the drop-down menu. You might need to go back and add classes to your cart after selecting the correct term.