A few times there has been some issues for some users with Microsoft Teams, that the users can’t see the calendar or can’t create events. One error message is
Microsoft.Exchange.Clients.Owa2.Server.Core.OwaUserHasNoMailboxAndNoLicenseAssignedException
On the admin panel -> user -> Mail you see message
This user doesn't have an Exchange Online license.
Based on the article I found on Microsoft.com, there were comments that helped me out.
Solution
Go the user admin panel, and remove all licenses. Then re-add the licenses.
Now iff you go to the Teams admin panel, and check the user -> Mail, you see now
We are preparing a mailbox for the user.