Thanks for your question.
Looks like a possible hiccup in the redemption process. After logging in to Founders Hub, go to the 'Benefits' tab and select Redeem in the Azure sponsorship benefits section. Use your Azure subscription email when redeeming credits.
See: https://learn.microsoft.com/en-us/microsoft-for-startups/benefits/azure-credits-billing
If you cant still redeem, I will recommend visiting support to assist.
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola