SharePoint only allows the creation of host-named site collections with a PowerShell command that must be executed on your server. Since ShareGate Migrate is a client-side application, it cannot execute such commands. In order to copy your host-named site collection, you must first create the site collection at the destination with the proper site template, then use ShareGate Migrate to merge your source site in your fresh site collection.

Promote doesn't support Host-Named Site Collections. You can use Copy Structure to merge your site in an existing site collection instead.