Page Caching is not available?

I recently made the switch from WP Super cache to this plugin W3 Total Cache. When I go to the plugin settings page, I get the following error box:

Page caching is not available: please add: define(‘WP_CACHE’, true); to /home/unlockmy/public_html/wp-config.php. This error message will automatically disappear once the change is successfully made.

So I go to my wp-config.php file, add the define(‘WP_CACHE’, true); to the end, in a new line following the last line require_once(ABSPATH . ‘wp-settings.php’);

When I go to the plugin settings again the error box doesn’t go away and still stays there say the same message. I don’t get it doesnt work after I added the line of code that they wanted me to. Thanks for the help.

https://www.ads-software.com/extend/plugins/w3-total-cache/