If a student gets this message, then the class invitation link that students use to create their accounts and enroll in their class and activate their license has expired or been deleted.
Usually, to solve this problem, the instructor should log into their account, click the three stacked dots to the right of their class, and choose the Invite Student: View/Edit Class Reservation option. Then they should click to edit the reservation, choose how many more licenses need to be reserved for the class, and edit the expiration date to be in the future (usually by a couple weeks), then Save. After they have done this, the student can try their link again.
See full guide to editing a class invitation link here, if additional guidance is needed: https://support.testout.com/hc/en-us/articles/4423172994715