Notice: Constant WP_POST_REVISIONS already defined

I have the same problem before.
I put WP_POST_REVISIONS in the end of wp-config.php file and it didn’t work correctly.
You should put your codes before defining ABSPATH and before this line:

/* That's all, stop editing! Happy blogging. */

it must be something like in the following:

define( 'WP_POST_REVISIONS', 6 );
/* That's all, stop editing! Happy blogging. */

/** Absolute path to the WordPress directory. */
if ( !defined('ABSPATH') )
    define('ABSPATH', dirname(__FILE__) . "https://wordpress.stackexchange.com/");

/** Sets up WordPress vars and included files. */
require_once(ABSPATH . 'wp-settings.php');

I hope it solves your problem.
So don’t put your codes in the end of wp-config.php file