In order to use CloudLinux's Ruby Selector to deploy Ruby applications to your cPanel server running CloudLinux, you'll need to ensure CloudLinux's Ruby packages.
To install Alt-Ruby, simply run the following commands:
yum groupinstall alt-ruby
yum install ea-apache24-alt_mod_passenger
Then, be sure to navigate to WHM > CloudLinux LVE Manager, then Options, and ensure that under "User Interface Settings" that "Hide Ruby App in web interface" is unchecked.
Finally, ensure your Feature List includes Setup Ruby App
For more information, please see CloudLinux's documentation.