Hi,
According to this document, to transfer the credits you can try logging into 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. You can raise request by traversing to this link: https://aka.ms/startupsupport
Kindly accept answer if it helps , Thanks!