This iPhone 6 is running iOS 10.1 (14B55c), which may not be supported by this version of Xcode

To use the iOS 10.1 beta you need to use the Xcode 8.1 beta. Xcode 8 doesn’t support the iOS 10.1 beta.

And now since iOS 10.1 and Xcode 8.1 are out of beta, be sure you update to the latest, non-beta of both.