You get the following warning in the promote summary details:

The user '<USER>' was not found and cannot be added as a site administrator

Because many SharePoint environments have been used for multiple years, there is a possibility that some site administrators and security groups no longer exist in your directory. For instance, if an administrator leaves the company, SharePoint will keep a reference to that user in your site settings. When the app tries to add that same user as an administrator in the new site collection, you get that warning message because the user or group cannot be found in your Active Directory.

Select a different site collection administrator in the promote options.