After adding a user into an on premise to Office 365 Mailbox or Calendar Synchronization collection, the following error is observed in the MAgE log and the mailbox is not being created:
Error Quest.MigrationManager.Exchange.O365UserProvisioning.O365UserLicenseException: Failed to set license for user - 'ExampleUser@domain.com': 'Invalid property: not allowed to have subscription when usage location is null or in unknown status.
This issue is encountered when the user in question has been added into an Office 365 Mailbox or Calendar synchronization collection, before they have been migrated by the Directory Migration Agent. When the user has been matched but not migrated, the Directory Migration Agent has not set the Country Code value on the associated Office 365 user object.
In order to have a mailbox created in Office 365, this property must be set (the bolded and italicized section of the log example) is the specific cause for this failure.