What is the error when creating a login form on Symfony [ERROR] Class "..." doesn't exist; please enter an existing full class name.?

Question

symfony

Here, according to this manual , I am trying to create a login form, but between these commands:

Choose a name for the controller class (e.g. SecurityController) [SecurityController]:
> SecurityController

Do you want to generate a '/logout' URL? (yes/no) [yes]:
> yes

I have another command:

Enter the User class that you want to authenticate (e.g. App\Entity\User) []:

and so that I don’t enter here, I get an error:

[ERROR] Class "lalala" doesn't exist; please enter an existing full class name.

Question: what should be entered here to go further?