Quick Cache not working

Hi all
when I enable quick cache it always says
“Failed to update your /wp-config.php file automatically. Please add the following line to your /wp-config.php file (right after the opening <?php tag; on it’s own line).”
<?php
define(‘WP_CACHE’, TRUE);

but i already add define(‘WP_CACHE’, TRUE); to wp-config.php already
and the bottom of my page Source doesn’t show quick cache note says it’s working

please help Thank you

https://www.ads-software.com/plugins/quick-cache/