This iPhone 8 (Model A1863, A1905, A1906, A1907) is running iOS x.0 (13B3333e), which may not be supported by this version of Xcode. (Version 9.x.x (9F2000))
Did see that before? So you need a Device Support files in order the Xcode would "speak" with your device.
You can download them here an paste into
/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport folder
To fast navigation go to Finder, press CMD+Shift+G and paste the route.