Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Since you are referring to Microsoft Business Center (MBC) and an MPSA (Microsoft Products and Services Agreement), this is separate from standard Microsoft 365 user administration. Microsoft documentation indicates that licenses purchased through MPSA are registered and managed through Microsoft Business Center.
Therefore, not having the Microsoft 365 Global Administrator role does not necessarily mean that you need to manage this user through standard Microsoft 365 user management.
If the Delete/Remove/Revoke Access option is not available under Account Management > Manage Access, I would suggest contacting Microsoft Volume Licensing Support for further assistance with the account association. A backend removal or unlinking cannot be performed through the Microsoft Community forum.
Since you mentioned that you cannot access the Microsoft 365 Admin Center, you can submit a Volume Licensing support request through the Microsoft Engage Center instead. Microsoft specifically provides this as an alternative support route when the Microsoft 365 Admin Center cannot be accessed.
Please refer to the official instructions here:
Contact Volume Licensing Support | Microsoft Learn
When submitting the request, please include the relevant agreement or Purchasing Account information, your current MBC role, the affected user's account/email information, and screenshots showing that the removal option is unavailable. Please avoid posting those details in this public forum, as they may contain personal or organizational information.
Just a quick note that this forum is a peer-to-peer community, so while members can share advice and experiences only, members do not have access to your organization, licensing account, or Microsoft backend systems. Therefore, please kindly contact the support team for the most efficient help.
I hope this information helps.