Adding Images into API

from the support forum of that plugin:

Your cpt may not be registered correctly. Try adding this to your functions.php (change “event” to your post type slug):

add_filter( 'register_post_type_args', function( $args, $post_type ) {

if( 'event' === $post_type ) {
    $args['show_in_rest'] = true;
}

return $args;

}, 10, 2 );

or make sure that you have show in rest active for your cpt when you actually registered it.