SDDM connection screen is missing


In KDE/plasma5: click system settings → in the list which appears there is no choice of connection screen (SDDM) as in the other distributions.

I have activated in the configuration file the following

# Enable the Plasma 5 Desktop Environment.
  services.xserver.displayManager.sddm.enable = true;
  services.xserver.desktopManager.plasma5.enable = true;

Do you think that it is possible to do something to make this choice appear?