I am running the most up to date version of Xcode (v10.1 (10B61)) on a High Sierra system (10.13.6 (17G10021)) and find that compiling for macOS will work, but not visually display in the sumulator.
I get the LLDB message: "unable to attach".
Advice online suggested submitting new security certificates, which I have done, but still the simulator will not show up, even though the code compiles.