It seems like you’ve downgraded to v0 Free plan. You can’t use the on demand credits on the v0 Free plan. You must have seen this warning when downgrading to v0 Free from a paid plan.
These credits are not wasted. You can upgrade to any paid plan and make use of them before they expire.
It is in place to avoid abuse of the paid plans. I’ll pass the feedback to our team about a student plan. But, for the foreseen future this is the only solution to use the on-demand credits you’ve left.