See the comment in qa-config.php (or qa-config-example.php), you need to define QA_CACHE_DIRECTORY in your config. Currently the folder needs to be outside the public root (in other words so it cannot be accessed in a web browser via example.com/cache/...).
Something like this in your qa-config.php should work:
define('QA_CACHE_DIRECTORY', __DIR__.'/../cache/');
Then that folder should be made writable by the server (chmod 777).