Hi there, welcome to the forums!
Please be aware that creating your MongoDB account using your GitHub account will not result in you receiving MongoDB student benefits through the GitHub Student Developer Pack. You simply need to enter your GitHub credentials at MongoDB Student Pack regardless of how you’ve created your MongoDB account.
Considering the error you’re encountering, I would recommend simply creating your MongoDB account without using your GitHub account but using the same email address that your GitHub account uses. Then, go to MongoDB Student Pack and enter your GitHub credentials to verify that you have active GitHub Student Developer Pack benefits. From there, you can complete the learning path on MongoDB University. You’ll be emailed your free certification voucher after completing the learning path.
Best of luck!