You try to migrate a subsite, and you get the following error in your migration report:
Cannot create a site at 'https://x.sharepoint.com/sites/x/x' because an unexpected error occurred.
You can get this error when your Subsite creation setting is turned off in your SharePoint admin center.
When you migrate a subsite and the Subsite creation setting is turned off, the migration tool tries to create a new subsite at the destination, and the setting will not allow it.
You have two options to solve this problem.
If you cannot bypass your organization's policy, you can migrate your subsites as site collections.
If you have access, you can turn on subsite creation in your SharePoint admin center.