I had the same problem in Xcode Swift Playgrounds (v. 15.3). I was using UIKit/PlaygroundSupport and setting a UIViewController subclass to the liveView. That UIViewController subclass was defined in the main file (not in the Sources folder). When I made that UIViewController subclass public was when I got this error and the playground would run but the live view wouldn't show. When I removed the "public" accessor, that error disappeared and the live view started showing again. I know that this isn't the same type of project as the OP's, but I figured there might be something related.
atem
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution