For our better research, please provide more information about this case:
- Do other users within your organization have the same issue?
- Do you have the same problem in Teams web client?
Someone from your organization might have changed the settings and accidentally removed the calendar app for your team. To fix the issue, you need to have an admin access for your team. Follow this guide to turn on the MS Teams calendar again.
- Sign into Microsoft Teams Admin Center as a Microsoft Office 365 administrator.
- On the left corner of the home screen, click Users, and choose your account.
- Select the Policy tab for your account.
- Go to Global (Org-wide Default) under the App setup policy section to show the list of apps.
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