Please open a support ticket about this.

Remember that you do need to be a student with a valid github account that has participated in this promotion, as well.

Everything worked for me when I signed up under my Student account. Note that I believe it uses AJAX and automatically submits when you enter your code. If it is giving you that error, everything went by fine. You should have received an email with credits awarded. I did need a card to activate my account;

Hi,

After entering you payment method, click on the drop-down near your profile picture and go to Settings. Click on Billing, and scroll down, you should see and input box that says promo code, and enter it and click apply. After that you should get a credit of the amount you have.

Thank you!