Windows Small Business Server and Windows Server Essentials are domain controllers. Storage Migration Service can't yet cut over from domain controllers, but it can inventory and transfer files from them.
https://video2.skills-academy.com/en-us/windows-server/storage/storage-migration-service/overview
Also mixing roles like that really isn't recommended. A better option may be to install the hyper-v role (as only role) on host, then stand up two virtual machines. One for active directory domain services and other one as file server.
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