Kde plasma login screen doesn't appears anymore

Hello to everyone.

I’ve just installed nixos. The problem that I have is that kde plasma does not start anymore. When NixOS is fully loaded I see the command prompt that asks me to insert login and password and nothing more happens. What could be wrong ? Below you can give a look at my configuration file. thanks.

{ config, pkgs, ... }:

{
  imports =
    [ # Include the results of the hardware scan.
      ./hardware-configuration.nix
    ];

  # Use the GRUB 2 boot loader.
  boot.loader.grub.enable = true;
  #boot.loader.grub.version = 2;
  boot.loader.grub.devices = [ "nodev" ];
  # Use the systemd-boot EFI boot loader.
  #boot.loader.systemd-boot.enable = true;
  boot.loader.efi.canTouchEfiVariables = true;
  boot.loader.grub.useOSProber = true;
  boot.loader.grub.efiSupport = true;

  # networking.hostName = "nixos"; # Define your hostname.
  # networking.wireless.enable = true;  # Enables wireless support via wpa_supplicant.

  # Set your time zone.
  time.timeZone = "Europe/Rome";

  # The global useDHCP flag is deprecated, therefore explicitly set to false here.
  # Per-interface useDHCP will be mandatory in the future, so this generated config
  # replicates the default behaviour.
  networking.hostName = "ziomario-nixos";
  networking.wireless.enable = false;
  networking.useDHCP = false;
  networking.interfaces.eno1.useDHCP = true;

  # Configure network proxy if necessary
  # networking.proxy.default = "http://user:password@proxy:port/";
  # networking.proxy.noProxy = "127.0.0.1,localhost,internal.domain";

  # Select internationalisation properties.
  i18n.defaultLocale = "it_IT.UTF-8";
  console = {
  font = "Lat2-Terminus16";
  keyMap = "it";
  };

  # Enable the X11 windowing system.
  services.xserver.enable = true;


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

  # Configure keymap in X11
  services.xserver.layout = "it";
  services.xserver.xkbOptions = "eurosign:e";

  # Enable CUPS to print documents.
  # services.printing.enable = true;

  # Enable sound.
  sound.enable = true;
  hardware.pulseaudio.enable = true;

  # Enable touchpad support (enabled default in most desktopManager).
  services.xserver.libinput.enable = true;

  # Define a user account. Don't forget to set a password with ‘passwd’.
  users.users.mario = {
  isNormalUser = true;
  extraGroups = [ "wheel" ]; # Enable ‘sudo’ for the user.
  };

  # List packages installed in system profile. To search, run:
  # $ nix search wget
  environment.systemPackages = with pkgs; [
     nano # Do not forget to add an editor to edit configuration.nix! The Nano editor is also installed by default.
     wget
     firefox
     rambox
     google-chrome
     virt-manager
     docker
     vlc
   ];

  # Some programs need SUID wrappers, can be configured further or are
  # started in user sessions.
  # programs.mtr.enable = true;
  # programs.gnupg.agent = {
  #   enable = true;
  #   enableSSHSupport = true;
  # };

  # List services that you want to enable:

  # Enable the OpenSSH daemon.
  services.openssh.enable = true;

  # Open ports in the firewall.
  # networking.firewall.allowedTCPPorts = [ ... ];
  # networking.firewall.allowedUDPPorts = [ ... ];
  # Or disable the firewall altogether.
  networking.firewall.enable = false;

  # This value determines the NixOS release from which the default
  # settings for stateful data, like file locations and database versions
  # on your system were taken. It‘s perfectly fine and recommended to leave
  # this value at the release version of the first install of this system.
  # Before changing this value read the documentation for this option
  # (e.g. man configuration.nix or on https://nixos.org/nixos/options.html).
  system.stateVersion = "21.05"; # Did you read the comment?

nixpkgs.config.allowUnfree = true;
nixpkgs.config.permittedInsecurePackages = [
         "rambox-0.7.7"
       ];
}