Add this to the functions.php file in your child theme’s folder
<?php
function my_et_builder_post_types( $post_types ) {
$post_types[] = 'fullpage';
$post_types[] = 'fullpage-section';
$post_types[] = 'fullpage-slide';
return $post_types;
}
add_filter( 'et_builder_post_types', 'my_et_builder_post_types' );
?>
Source: https://www.elegantthemes.com/blog/divi-resources/how-to-add-the-divi-builder-to-custom-post-types-divi-nation-short
