I did it with this code!
/*save custom post field*/
function my_acf_save_post( $post_id ) {
// An array of IDs of categories we want this post to have.
$cat_ids = get_field('localita-popolata');
//If this was coming from the database or another source, we would need to make sure
//these were integers:
$cat_ids = array_map( 'intval', $cat_ids );
$cat_ids = array_unique( $cat_ids );
wp_set_object_terms( $post_id, $cat_ids, 'struttura_category' );
}
add_action('acf/save_post', 'my_acf_save_post', 20);