This answer should be more of a comment against Dawn Song's comment earlier, but since I don't have enough reputation, I'm going to write it as an answer.
According to the forum page
https://forums.developer.apple.com/thread/11313
"In general, you should never just delete the CoreSimulator/Devices directory yourself. If you really absolutely must, you need to make sure that the service is not runnign while you do that. eg:"
# Quit Xcode.app, Simulator.app, etc
sudo killall -9 com.apple.CoreSimulator.CoreSimulatorService
rm -rf ~/Library/*/CoreSimulator
I definitely ran into this issue after deleting and reinstalling Xcode.
You might encounter a problem trying to connect the build to a simulator device. The thread also answers what to do in that case,
gem install snapshot
fastlane snapshot reset_simulators