Mac OS X 10.9 Mavericks (Ruby 2.0) and Sencha Cmd, how to work together
The free upgrade Mavericks for Mac OS-X includes Ruby 2.0.
Now Sencha Cmd version 4.0 and below don't support that Ruby version.
Therefore you can't build Sencha Touch and Ext JS projects; since Sass/Compass is included in the build process and requires Ruby 1.8 or Ruby 1.9.
How to solve?
There are great blog posts online, such as: http://moduscreate.com/sencha-cmd-not-working-in-os-x-10-9-mavericks/ how to downgrade Ruby @ Mavericks. However there is a much easier and "better" way of solving this problem and that is upgrading your Sencha Cmd to the 4.0.1.x version.
Currently this version is in beta; but you can get exist to it via Sencha Cmd on the command-line.
Just run the following command:
sencha upgrade -b
It will install 4.0.1.x beta. After the installation, restart your terminal.
And go ahead and run a Sencha build!