You have just moved a user to a new tenancy and migrated the old domain over. When you sign into Mobile devices , its still trying to use the old email or oldtenancy onmicrosoft.com address

Go into the Microsoft Authenticator and remove the Authenicator account from the old tenancy and try again. If not there should be an option in Outlook to “Sign into another account”

On Windows this is a a bit more long winded.

1. Remove the old tenancy account from Accounts \ Work School Account
2. Unlink Onedrive from the old tenancy and sign into the new tenancy which should add the new tenancy into the Work School Account
3. Unsign out of Office apps and Resign in
4. Use Credential Manager to clear out the 365 Cached Credentials
5. If for some reason it still tries to login to old tenancy when you put the new email address tenancy in , use the new tenancy’s OnMicrosoft account which should reask for the username