Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
I tried to look this up, but I did not come across a solution.
Microsoft does not seem to provide support for the signup, and they do not tell you the reason why you failed.
There might be a possibility of a previous redemption (you are only allowed one), or you verified once in the past that recorded it as a redemption - but you did not activate the usage. Regardless it does not solve your problem.
My suggestion is to check with your school if they found a solution to this, if they require you to use this for your course. Likely you are not the only student who has faced this in the past, and what is their solution.
What I found online, not what you would like to hear, seems to be some people ended up using a paid subscription, but keeping the consumption small.
Hope others have better ideas.