After installing Xcode 10, especially when Xcode 9 is also installed, It has been observed that at times simulator fails to launch while running a project. Following steps can resolve the Issue
- Clear derived data Xcode -> Preferences -> Location -> Derived data
- Open edit schemes and make sure for Run an executable is selected. If the executable is set to None, as shown in the image below, the simulator will not launch.
- Restart mac