If you are running Xcode 7.3.1 (the previous version of Xcode) and you try to install an app to your iPhone running iOS 10 (latest version of iOS), you will most probably run into this issue.
It’s not a very helpful message at all. Doesn’t really say what’s actually wrong here. It appears that the problem is that this version of Xcode doesn’t know how to run an app on an iOS version that it’s not aware of. Xcode 7 does not support iOS 10.
To fix this, you’d need to download the latest version of Xcode which has support for iOS 10 and use the Device Support files from there. The biggest hurdle is the 4+ GB download, but once you’ve downloaded the latest Xcode (yes, you can have 2 versions of Xcode at the same time, just rename them appropriately in your Applications folder), you can create a symbolic link from the latest Xcode into your previous Xcode.
I renamed my previous version of Xcode to ‘Xcode 7.3.1’, so the command to create the symbolic link, in my case, looks like this :-
ln -s /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/10.0\ \(14A345\) /Applications/Xcode\ 7.3.1.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/10.0
Once you do this, you need to restart Xcode and you should now be able to install your app on your phone. In case, you get the error ‘Device is locked’, eve if your device is unlocked, you can try to unplug your device, restart Xcode and plug your device back in. It happened to me once and I was able to use the device following these steps.