In order for the Flatsome UX Builder to be available on CartFlows steps, the following function needs to be added to your child themes function file.
// Do not include this if already open!
/**
* Code goes in theme functions.php
*/
add_action( 'init', function () {
if( function_exists( 'add_ux_builder_post_type' ) ) {
add_ux_builder_post_type( 'cartflows_step' );
}
} );
Alternatively you can use the free Code Snippets plugin to add custom functions to your website.
Here is a tutorial on how to add custom functions to your website.