Hi and welcome to the forums!
If you successfully verified that you have active GitHub Student Developer Pack benefits on MongoDB Student Pack then you likely received a 50% discount voucher because your MongoDB University email address is different from your GitHub email address. If you go to mongodb.com/students, you should see a section on the right with the header ‘Claim Your Free MongoDB Certification’. In that section, there is a link to a form where you can request your free certification voucher.
As a note, the 100% discount voucher request form is only meant to be available to students who have successfully verified that they have active GitHub Student Developer Pack benefits.
Users who bypass our process and fill out the form after receiving a direct link from a classmate will be flagged as ineligible for a 100% discount voucher. This doesn’t apply to you, OP, since you’ve successfully activated your GitHub Student Developer Pack benefits; I’m just putting this info here for others who might be having the same issue.
I hope this answers your question, please follow-up if you have any others!