Have you verified that you have an Exchange Online license?
If that's okay, you could do the following process.
Close the Teams application.
Open the following location and delete all contents. Restart Teams.
%appdata%\Microsoft\Teams
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I have licence for 365 Business Standard, and there was no issue.
But, today, I cannot load calendar, and I cannot make a new meeting suddenly.
Please let me know the solution..
Have you verified that you have an Exchange Online license?
If that's okay, you could do the following process.
Close the Teams application.
Open the following location and delete all contents. Restart Teams.
%appdata%\Microsoft\Teams
Hi MyeongGangHeo-8980!
If you are the Office 365 admin, you can try to check your license by following these steps:
I did a test in my environment and the result like the following picture if you don't have Exchange Online license