Based on your description, it is recommended that you go through the corresponding troubleshooting according to the documentation:
Step 1: Verify that Teams Calendar App is enabled.
Step 2: Verify Teams upgrade Coexistence mode allows Teams meetings.
Step 3: Verify that Teams isn't blocked from accessing EWS for the entire organization.
Step 4: Verify that Teams isn't blocked from accessing EWS for the affected user.
Step 5: Verify that the Microsoft Teams Calendar App test is successful.
Step 6: Escalate the issue.
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