If you have a student license, you should make sure of the following:
1) Your MathWorks account is associated with your license, and 2) Your license is current
To check item (1), make sure you are signed in to Mathworks.com, and click on your username at the top right of the page. From there, you should see a menu drop down, with one of the options being "Associate license". Select that, and you will be taken to a new page, which will ask for your license number or activation key. You should have received either one when you purchased your license. Simply put that number in the box and follow the rest of the instructions to finalize your association.
NOTE: If you click on "My Account" when you see the drop down menu from your name on the top right, this will tell you what licenses you are already associated with.
Item (2) refers to how current you are with your license. This is based on the Software Maintenance Service, which you can read about at https://www.mathworks.com/services/maintenance.html. This will renew your subscription, and hence your access to MATLAB Online.
Please let me know if this helps.