xcode 9 simulator error unable to boot the simulator launchd failed to respond mac os sierra

I observed these issues sometimes when my mac is not connected to the internet. So here is a solutions list one of them may solve your problem.

1. Check if you have an internet connection running in your mac.
2. Restart the simulator.
3. Restart mac if required.

Hope this will help you. Thanks

Restarting Mac solved it for me.