So if you are like me, I have several versions of Xcode. If you try to create the Hello World app, rake will complain:
ERROR! You are using RubyMotion Starter. Only iOS 10.1 is supported in this release. If you would like to target older or newer (in beta) versions of iOS you can purchase a paid subscription.
You can try to first do:
sudo motion update
Solution seems to me is simpler than that. The starter restricts you to a current iOS version.
rake is wanting 10.1 in the message, so do xcode-select for the version where 10.1 is the current sdk version:
$ sudo xcode-select -s /Applications/Xcode.app/Contents/Developer/
and in this case, that Xcode.app is 8.1 so sdk version is 10.1