Never mind. This question is incorrect perspective.

I don't know if this is that same problem, but I was trying to edit a file Foo.swift and the preview wouldn't work. It complained about another file BarTest.swift not being in any target. Now BarTest.swift was a test file in a package that was being included in my project. It didn't have anything to do with anything happening in Foo.swift, BUT I did have it up in a tab in my Xcode editor. When I closed that tab, the error went away and I was able to preview again. This was in Xcode 12.2.