Same problem. Reinstallation via npm didn’t help.
Cordova CLI: 6.3.1
Gulp version: CLI version 1.2.2
Gulp local: Local version 3.9.1
Ionic CLI Version: 2.0.0
Ionic App Lib Version: 2.0.0
ios-deploy version: 1.8.6
ios-sim version: 5.0.8
OS: Mac OS X El Capitan
Node Version: v6.4.0
Xcode version: Xcode 7.3.1 Build version 7D1014
UPD Solved
By following Installing Ionic instructions
Tried to install previously on Desktop, not sure whether that’s the reason or not for fail installation.