Azure Migrate isn't designed to migrate just a file share to an Azure File Share; it migrates entire servers (virtual machines) to Azure. For migrating an on-premises file share to an Azure VM, you can use Azure Migrate to migrate the entire server that hosts the share, or you can use specialized tools like Azure Storage Mover, Robocopy, or AzCopy for direct data migration to an Azure file share.
Reference:
https://docs.azure.cn/en-us/migrate/common-questions-server-migration