@Alex , Thanks for posting in Q&A. From your description, it seems you are accessing Microsoft 365 admin portal. but get permission issue. Based on my researching, global administrator can access Microsoft 365 portal.
But from your description, it seems you already have global administrator role. You can check if Microsoft 365 license has also assigned to your account. If yes, then I suggest you contact Microsoft 365 support in the following link to look into the issue:
https://learn.microsoft.com/en-us/microsoft-365/admin/get-help-support?view=o365-worldwide
Hope it can help.
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