Could not load database driver mysqli! on opencart

You seem to be missing the following line in your config.php file which appears usually after define('DIR_SYSTEM', '../system/');

define('DIR_DATABASE', '../system/database/');

I would go back to relative paths, keep in mind there are two config files, one in the root of the site and one in admin. You need to add the '../system' in the admin one if the one in root is 'system'