Hi @Ned ,
You can try this way as an admin:
Go to https://tenant.sharepoint.com/sites/your-site-collection/\_layouts/15/sitemanager.aspx.
Select the subsite from the Treeview >> Click on “Move” from the subsite’s context menu.
Select the target subsite and click “OK” to move the subsite to the new subsite.
And then, the migration succeeds.
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