I start the tutorial (as newbie) and everythings works fine till:
http://symfony.com/doc/current/book/page_creation.html#creating-a-page-route-and-controller at step
If you're using apache server with Symfony 4 (not the Symfony server) just add :
composer require symfony/apache-pack
Don't forget to address your Document Root to
where/is/your/symfony/public
in apache configuration.