I am currently using Avast Security for Mac and have discovered an issue / conflict between Avast and XCode simulator.
This stackoverflow post (from another user) describes the issue: https://stackoverflow.com/questions/46980164/cant-visit-website-on-ios-simulator-this-connection-is-not-private
Essentially, in order to use XCode simulator I have to disable Avast WebShield. This is obviously not ideal, and indeed could force me to look at alternate AV Software if it cannot be address.