Flatsome UX Builder?

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.