以下是一个老外的回答：
This happened to me after installing a build from TestFlight and overwriting it with the debug build from Xcode.
Nothing helped, no clean build, no Derived Data voodoo …
How I finally got it working was:
- Delete debug build from phone (and watch if it’s still there).
- Download TestFlight build again.
- Install Test Flight Apple Watch app build from TestFlight app.
- Delete TestFlight Watch app build from Phone using the TestFlight app.
- Delete the TestFlight iOS app build from the phone
- Now everything seems back to normal. I can run the debug build from Xcode.
but一般来说不用这么复杂。只要把你的设备上的debug的app删除了就可以。如果删除了还是不行，再按照他的方法试一次。