Please open target tenant Sharepoint Admin and enable subsites creation option, as per this article:
https://learn.microsoft.com/en-us/sharepoint/manage-site-creation#manage-detailed-site-and-subsite-creation-settings-in-the-classic-sharepoint-admin-center
Once the changes have been performed on the tenant, retry the migration. If the problem continues, please try the following:
Once completed retry the migration.
