Slim Application Error (The system cannot find the path specified)

<?php

require '/vendor/autoload.php';
date_default_timezone_set ('Asia/Jakarta');

// $log = new Monolog\Logger('name');
// $log->pushHandler(new Monolog\Handler\StreamHandler('app.log', Monolog\Logger::WARNING));
// $log->addWarning('Foo');

//call
$app = new \Slim\Slim(array(
    'view'=> new \Slim\Views\Twig()
    ));

$view = $app->view();
$view->parserOptions = array(
    'debug' => true
    );

$view->parserExtensions = array(
    new \Slim\Views\TwigExtension()
    );

//define
$app->get('/', function () use($app){
    $app->render('about.twig');

})->name('home');
$app->get('/contact', function() use($app){
    $app->render('contact.twig');
})->name('contact');

$app->post('/contact', function() use ($app){
    //var_dump($app->request->post());
    $name= $app->request->post('name');
    $email= $app->request->post('email');
    $msg = $app->request->post('msg');

    if(!empty($name) && !empty($email) && !empty($msg)){
        $cleanName = filter_var($name, FILTER_SANITIZE_STRING);
        $cleanEmail = filter_var($email, FILTER_SANITIZE_EMAIL);
        $cleanMsg = filter_var($msg, FILTER_SANITIZE_STRING);

    }else{
        //message the user that there was the problem
        $app->redirect('/contact');
    }

    $transport= Swift_SendmailTransport::newInstance('/usr/sbin/sendmail -bs');
    $mailer= \Swift_Mailer::newInstance($transport);

    $message = \Swift_Message::newInstance();
    $message-> setSubject('Email From Our Website');
    $message-> setFrom(array(
            $cleanEmail => $cleanName
        ));
    $message-> setTo(array(
            'wnchstdean@gmail.com'
        ));
    $message-> setBody($cleanMsg);

    $result = $mailer-> send($message);

    if($result > 0){
        //send a message that says thank you, because the message is succeed
        $app-> redirect('/');
    }else{
        //send a message to the user that the message fail to send
        //log that there was an error
        $app-> redirect('/contact');
    }


});
//run
$app->run();