Once, I found that the project had multiple targets. And the file wasn't a member of the currently selected target. But that only happened once. Other times, I found that repeatedly clicking the preview button (or repeatedly pressing Option-Cmd-P) fixed it. But most of the time, the preview is simply not appearing. Xcode 12.0 and 12.2 series are rough for me, right now.

After upgrading to Xcode 12.2 beta 3 (12B5035g), one failing preview offered an error: a unittest (i.e. an XCTestCase subclass) was not part of any target. Of course, it was part of the unittest target only. So I have no idea why the preview depended on that unittest file. I just deleted the file from Xcode, and for this project, the error "Failed to build ***.swift" keeps popping up above the preview but if I repeatedly hit Cmd-Shift-P, it does render. Edit: never mind, other views no longer render. Very frustrating.

The problem cropped up again, and I bit the bullet. In Xcode, I created a new project and copied all the source from the old to the new project. I recreated everything in the new project, meaning: set the bundle ID, version number, build number, launch screen et cetera. That fixed it. So, it's not the source files themselves, it's something in the project file(s). I did a manual diff between the <projectname>.xcodeproj/project.pbxproj of the old and the new project, but I gave up after staring 15 minutes at the incredibly messy diff. Preview now work.