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。