It seems if you are using generic student accounts in the tenant? If yes the account once used for registration cannot be reused, if you think that is incorrect please raise a support ticket with Microsoft to investigate this issue - https://learn.microsoft.com/en-us/azure/cost-management-billing/manage/troubleshoot-azure-sign-up
Also you can try to register with another account that is unique and never been used.
Hope this helps.
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