[Hummingbird] Hummingbird cache setup problems

I'm setting up Hummingbird for first time. I've remove WP Super Cache plugin. I've checked .htaccess file is set at 644 permissions. I have checked that "define('WP_CACHE', true); " is in my wpconfig.php file. I have VPS hosting with Dreamhost which is Apache. The website is not linked to Cloudflare.
This is the error I am getting

Hummingbird could not locate the WP_CACHE constant in wp-config.php file for WordPress. Please make sure the following line is added to the file:
define('WP_CACHE', true);