iPhone 5s running ios 12.1 not supported in xcode

Update Xcode to the latest version on the app store. Had this issue as well.

Quicker/Smaller Fix

If you don't want a big Xcode update due to limited storage/time then you can manually download the required support files instead of the whole update.

I was on Xcode 9.4 so I downloaded the files from here and pasted in /Applications/Xcode/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport