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 kept getting this error and didn't know why after searching all the other questions here, took forever to figure this out.
The solution was really simple. A while back I had removed a few files and re-added them to the extension, but they didn't get added back to compile sources.
Added the exention's .m files back into it's compile sources and the error went away.
Very misleading error message.