Log in to the Microsoft for Startups Founders Hub
Go to the Benefits tab and select Redeem in the Azure sponsorship benefits section.
If you still have issues, I would suggest you reach out to Microsoft for Startups support team for this. See: https://aka.ms/startupsupport
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola