Some users may be unable to see the Calendar icon in the Microsoft Teams sidebar ID: TM1037342 - FIX

See below the suggested fix for the calendar app missing or other apps missing in Teams

1. Sign out of teams.
2. Wait for it to finish logoff and it will respawn.
3. Fully close teams (Find Teams in the taskbar area near the clock and right-click Quit it)
4. Delete contents of:  "%userprofile%\appdata\local\Packages\MSTeams_8wekyb3d8bbwe\LocalCache\Microsoft\MSTeams"
5. Launch Teams

If the above fix does not fix it, perform a RESET (See image below)