My bad, i copied and pasted the wrong code. ( i said my custom field is “category_image” but really is “immagine_categoria” same thing…)
$category_image = get_field('immagine_categoria', 'custom_catalog_category_39');
$size = "page-header";
$imgURL = wp_get_attachment_image_src( 39, $size);
var_dump($imgURL);
This returns bool(false)