Restart both Iphone and Apple Watch. After restarting, make sure your both devices are unlocked then try again to run the Xcode project. if this don't help, switch off Apple Watch and try to run the Xcode project again for Iphone. If this don't help, make sure you haven't changed your Apple Id password recently. In case you did so, try to login from Xcode - Preferences - Account.
Trying all those mentioned above, helped me.
iWill_lms
Users receive reputation points by contributing quality content to the forums.
Posts include post and replies published on the forums.
Post authors can mark replies as Accepted to indicate successful solutions.
Apple Developer Forums admins can mark replies as Apple Recommended to indicate an approved solution