iOS 12 not supported by Xcode 9.4 : Could not locate device support files
iOS 12 is only supported by the beta of Xcode 10 or higher.
If you want to use your iPhone 8 with this iOS version (NOT RECOMMENDED) with your Xcode 9.4 you can try to download the last beta of Xcode 10 and after connecting the iPhone to the mac go to this folder:
/Applications/Xcode10.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport
You can see the iOS version folder of the iPhone, copy and past it to the same location in your Xcode 9.4 folder.
After this you should be able to deploy to your iPhone 8.
As answered by @Kerberos you can copy the iOS 12 device support files to:
/Applications/Xcode10.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport