Xcode 10 Could not find any available simulators for iOS when install Carthage dependencies
What worked for me was uninstalling Carthage and installing it again.
brew uninstall carthage --forcebrew install carthage
Upgrading to v0.31.1 or v0.31.2 fixes it:
brew update && brew upgrade carthage
After installing Xcode 10.1, I had trouble even with Carthage 0.31.2. But I also have a habit of deleting all simulators when installing Xcode major versions. Adding a simulator for iOS 12.1 fixed things.