Error :Process could not be started [The system cannot find the path specified. ] in slim

here is my index.php source code <?php require DIR . '/vendor/autoload.php';

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

$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(), );

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

$app->get('/home', function() use($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){ $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 {
    $app->redirect('contact');
}

$transport = Swift_SendmailTransport::newInstance('/usr/sbin/sendmail -bs', 465);
$mailer = \Swift_Mailer::newInstance($transport);

$message = \Swift_Message::newInstance();
$message->setSubject('Email From Our website');
$message->setFrom(array(
    $cleanEmail  => $cleanName
));
$message->setTo(array('amr.aly57@gmail.com'));
$message->setBody($cleanMsg);

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

if($result > 0) {
    //Thank you
    $app->redirect('/');
} else {
    //error

    $app->redirect('contact');

}

});

$app->run();

And when i send message i get this error : Swift_TransportException Message: Process could not be started [The system cannot find the path specified. ] File: C:\wamp\www\treehouse\vendor\swiftmailer\swiftmailer\lib\classes\Swift\Transport\StreamBuffer.php Line: 298