Do you have any operations before this issue occurred?
Do you have the same issue in other Teams channels?
To troubleshoot your issue, we recommend you check from these aspects:
- check that user has license for Exchange online
- check that Calendar app is enabled in the default Global policy of Teams .
- check that the user is using/has applied this Global policy
- log out and back in, waiting a day, forcibly log him out from the admin center etc.
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