Hope you are doing well.
Per my research, there can be multiple things that you can check at your end-
- Is your Office 365 subscription expired?
- Check if your user has a valid Teams license?
- Is service enabled for Microsoft Teams for that particular user?
However, I would suggest you to raise a support case with Microsoft Graph, a Support Engineer will be able to assist you better. You can raise support ticket from http://aad.portal.azure.com/ or https://admin.microsoft.com/#/support/requests.
Alternatively, you can look into a similar thread.
Hope this helps.
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