Symfony, CMS tutorial, do not understand where is my mistake

[gintare]

I am following online tutorial for Symfony CMS: http://symfony.com/doc/master/cmf/tutorial/introduction.html

I am getting the error:
An exception has been thrown during the rendering of a template ("The menu "main" is not defined.") in src\Acme\BasicCmsBundle\Resources\views\Default\pa ge.html.twig at line 4.
500 Internal Server Error - Twig_Error_Runtime
1 linked Exception:
InvalidArgumentException »


I am on the page about menu
http://symfony.com/doc/master/cmf/tutorial/the-frontend.html


Where is the mistake?

<?php
 
// src/Acme/BasicCmsBundle/DataFixtures/PHPCR/LoadPageData.php
namespace Acme\BasicCmsBundle\DataFixtures\PHPCR;
 
use Acme\BasicCmsBundle\Document\Page;
use Doctrine\Common\DataFixtures\FixtureInterface;
use Doctrine\Common\Persistence\ObjectManager;
use Doctrine\ODM\PHPCR\DocumentManager;
 
class LoadPageData implements FixtureInterface
{
    public function load(ObjectManager $dm)
    {
        if (!$dm instanceof DocumentManager) {
            $class = get_class($dm);
            throw new \RuntimeException("Fixture requires a PHPCR ODM DocumentManager instance, instance of '$class' given.");
        }
 
        $parent = $dm->find(null, '/cms/pages');
 
        $rootPage = new Page();
        $rootPage->setTitle('main');
        $rootPage->setParentDocument($parent);
        $dm->persist($rootPage);        
 
        $page = new Page();
        $page->setTitle('Home');
        //$page->setParentDocument($parent);
        $page->setParentDocument($rootPage);
        $page->setContent(<<<HERE
Welcome to the homepage of this really basic CMS.
HERE
        );
 
        $dm->persist($page);
 
        $page = new Page();
        $page->setTitle('About');
        $page->setParentDocument($rootPage);
        $page->setContent(<<<HERE
This page explains what its all about.
HERE
        );
        $dm->persist($page);
 
        $dm->flush();
    }
}
 
 

{# src/Acme/BasicCmsBundle/Resources/views/Default/page.html.twig #}
<h1>{{ page.title }}</h1>
 
{{ knp_menu_render('main') }}
 
<p>{{ page.content|raw }}</p>
<h2>Our Blog Posts</h2>
<ul>
    {% for post in posts %}
        <li><a href="{{ path(post) }}">{{ post.title }}</a></li>
    {% endfor %}
</ul>