How to resolve iPhone 7 running iOS 13.1.2 not supported by Xcode 10.1?
You need to have Xcode 11 installed for this to work. Xcode 10.x won't recognize iOS 13.

Try logging out and in of App Store app on your Mac. If it doesn't work you try restarting your Mac. If that fails too, attempt a reinstall of Xcode from the App Store.