should go in your functions.php file
there is a missing ) in the code which is the cause of the 500
add_filter('acf/save_post', 'convert_classes_to_standard_wp', 20);
function convert_classes_to_standard_wp($post_id) {
// use a different field name for your converted value
$meta_key = 'converted_classes';
// clear any previously stored values
delete_post_meta($post_id, $meta_key);
// get new acf value
$values = get_field('classes', $post_id);
if (is_array($values) && count($values)) {
foreach ($values as $value) {
add_post_meta($post_id, $meta_key, $value, true);
} // end foreach
} // end if
}
