Symfony – AbstractFormLoginAuthenticator problem

I am building a very simple login page using Symfony framework.
I added

use SymfonyComponentSecurityGuardAuthenticatorAbstractFormLoginAuthenticator;

and called it as:

class LoginFormAuthenticator extends AbstractFormLoginAuthenticator implements PasswordAuthenticatedInterface

in my LoginFormAuthenticator.php but when I go to localhost, it says :

Attempted to load class “AbstractFormLoginAuthenticator” from namespace “SymfonyComponentSecurityGuardAuthenticator”.
Did you forget a “use” statement for another namespace?

What can be a problem here knowing that I called right class from right namespace, as it is declared in Symfony documentation?

Thank you in advance.

I expected to view my login page as everything is done regarding to Symfony documentation. Instead of that I am getting very bad error.
Screenshot