error: WatchKit Extension doesn\'t contain any WatchKit apps. Verify that the value of WKWatchKitApp in your WatchKit App\'s Info.plist is set to YES.
I was able to fix this error by removing the extension's binary from "Embedded Binaries" in Main App's "General Settings".
I think that was still in there from the time I created the watch-target, which was for Watch OS 1.