This error is commonly caused by an unlicensed mailbox account. Check that all of the migrating mailboxes are licensed in Microsoft 365 first.
If a mailbox account has been assigned a license and the error persists, it is usually caused by a non-Null value in the "msExchMailboxGuid" attribute of the mailbox account.
By default, when synchronizing a user from an on-premises environment that has a mailbox assigned to it, the value of the "msExchMailboxGuid" attribute will also be synced. Assigning an Microsoft 365 license in this case will not result in the creation of a mailbox.
Follow these steps to confirm the issue in Microsoft 365:
- Log in to the Microsoft 365 tenant using a Global Admin account, and go to the Admin page.
- Under the Active Users panel, search for the email address of the affected user.
- Click on the user name, and look under the Mail Settings.
- The message associated with the mailbox will be :"This user's on-premises mailbox hasn't been migrated to Exchange Online. The Exchange Online mailbox will be available after migration is completed."
Follow the "Pre-Migration tasks" instructions in the [Azure for MigrationWiz](Filtering objects using Azure Active Direct (AAD) Connect) article. This will allow mailbox to be created in Microsoft 365 so that MigrationWiz can proceed with the migration.