Can't install WatchKit app on Apple Watch
A friend who works on the USA Today iOS team, including the Apple Watch app, suggested I go to the project settings for the WatchKit App target and set "Skip Install" to NO. Sure enough, after doing this, I was able to install.
How to install app on apple watch with my version of OS?
While your phone has the latest iOS that it supports, your watch doesn't - A stand alone watch app requires watchOS6 or later.
To deploy a stand-alone app you need to change the deployment target of your watch app to watchOS 6 and upgrade your watch to watchOS6 (and your phone to iOS 13).
Since your phone doesn't support iOS 13, the alternative is to develop a watch app that has a companion iOS app; this is supported on your current iOS and watchOS versions.
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