A Microsoft 365 backup fails with the error "User's mysite not found". This can happen for OneDrive, Exchange, SharePoint, or Teams.
This happens because the account's user has not yet logged into their account. As such, the account has not been provisioned by Microsoft and cannot be backed up.
To resolve this, ensure that both of the following requirements are met:
The account must be licensed correctly (e.g. not Personal or Family Edition)
The account must be provisioned, i.e. the user must have logged into the specific product with the relevant account. Backups will not work if the account has not been provisioned, even if it has the correct licence.