I managed to get round this by copying Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/13.4 from the latest Xcode.xib into my installed X

I managed to get round this by copying Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/DeviceSupport/13.4 from the latest Xcode.xib into my installed Xcode package. I can now debug 13.4 devices on Xcode 11.3.