Phalcon INVO Application - Volt directory can't be written

Hi, Iam a beginner in php development. I started learning phalcon framework and was working on the examples provided on the site. Everything was working good then when i started learning the tutorial 2 that is INVO Application, downloaded the complete project and placed in the public html folder and when i executed the APP from localhost its saying Volt directory can't be written. I didnt even change anything in the file, still i believed atleast the homepage will show up but nothing showed up.

what to do? pls help.

$di->set('view', function() use ($config) {

$view = new View(); $view->setViewsDir(APP_PATH . $config->application->viewsDir); $view->registerEngines(array( ".volt" => 'Phalcon\Mvc\View\Engine\Volt' ));
return $view; });

$di->set('volt', function($view, $di) { $volt = new VoltEngine($view, $di);
$volt->setOptions(array( "compiledPath" => "invo/cache/volt/" ));

$compiler = $volt->getCompiler(); $compiler->addFunction('is_a', 'is_a'); return $volt;
}, true);