You probably need to use a Joomla import to load in the Joomla API you need. CB doesn't need Joomla API except core functionality, which is always present as CB has its own API.
To utilize Joomlas content helper using CB API only you can do the below (I use this in CB Articles and CB Blogs, confirmed working).
Code:
require_once ( $_CB_framework->getCfg( 'absolute_path' ) . '/components/com_content/helpers/route.php' );
Ensure the global $_CB_framework is added in your function.