In reply to @ahdyt:matrix.org
It's basically just upgrade if there's a new update, and to create new update, either you have to update channel or flakes first.

In reply to @jonringer:matrix.org
try sudo nix-channel --update

In reply to @jonringer:matrix.org
did you try NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs/archive/refs/heads/master.tar.gz nix-shell -p steam --run steam?

here is my configuration.nix

# Edit this configuration file to define what should be installed on
# your system.  Help is available in the configuration.nix(5) man page
# and in the NixOS manual (accessible by running ‘nixos-help’).

{ config, pkgs, ... }:

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

  # Laptop GPU using NVIDIA-PRIME
  services.xserver.videoDrivers = [ "nvidia" ];
  services.xserver.dpi = 100;
  hardware.nvidia.modesetting.enable = true;
  hardware.nvidia.prime = {
    sync.enable = true;

    # Bus ID of the NVIDIA GPU. You can find it using lspci, either under 3D or VGA
    nvidiaBusId = "PCI:1:0:0";

    # Bus ID of the Intel GPU. You can find it using lspci, either under 3D or VGA
    intelBusId = "PCI:0:2:0";
  };

  # Use the GRUB 2 boot loader.
  boot.loader.grub.enable = true;
  boot.loader.grub.version = 2;
  # boot.loader.grub.efiSupport = true;
  # boot.loader.grub.efiInstallAsRemovable = true;
  # boot.loader.efi.efiSysMountPoint = "/boot/efi";
  # Define on which hard drive you want to install Grub.
  boot.loader.grub.device = "/dev/sda"; # or "nodev" for efi only

  networking.hostName = "nixos"; # Define your hostname.
  # networking.wireless.enable = true;  # Enables wireless support via wpa_supplicant.
  networking.networkmanager.enable = true;
  programs.nm-applet.enable = true;

  # Set your time zone.
  time.timeZone = "Asia/Jakarta";

  # 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.useDHCP = false;
  # networking.interfaces.enp0s3.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 = "en_US.UTF-8";
  # console = {
  #   font = "Lat2-Terminus16";
  #   keyMap = "us";
  # };

  # DE setup
  services.xserver = {
    enable = true;
    windowManager.i3 = {
        enable = true;
        package = pkgs.i3-gaps;
        extraPackages = with pkgs; [
            dmenu
            i3status
            i3lock
            volumeicon
            clipit
            xfce.xfce4-power-manager
            alacritty
            i3lock-fancy
            j4-dmenu-desktop
            xautolock
            playerctl
        ];
        configFile = "/etc/nixos/configs/i3-config";
    };
    desktopManager.xfce.enable = true;
    displayManager.lightdm.enable = true;
  };
  

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

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

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

  # Enable proprietary stuff
  # Used for microsoft fonts and drivers
  nixpkgs.config.allowUnfree = true;

  # Enable fonts
  fonts.fonts = with pkgs; [
    noto-fonts
    noto-fonts-cjk
    noto-fonts-emoji
    liberation_ttf
    fira-code
    fira-code-symbols
    mplus-outline-fonts
    dina-font
    proggyfonts
    corefonts
    vistafonts
  ];

  # 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.notroot = {
    isNormalUser = true;
    extraGroups = [ "wheel" "networkmanager" ]; # Enable ‘sudo’ for the user.
  };


  programs.steam.enable = true;
  environment.systemPackages = with pkgs; [
    wget vim
    procps
    polkit_gnome

    # User land stuff
    brave
    gparted
    xfce.mousepad
    xfce.terminal
    lxrandr
    libreoffice-fresh
    vscodium
    git
    dbeaver
    postman
    element-desktop
    discord
    megasync
    cherrytree
    htop
    p7zip
    mate.engrampa
    blender

    # To fix screen resolution and stuff
    arandr
  ];

  programs.bash.shellAliases = {
    mc = "source ${pkgs.mc}/libexec/mc/mc-wrapper.sh";
  };

  # List services that you want to enable:

  # polkit stuff
  security.polkit.enable = true;
  systemd.user.services.polkitGnome = {
    enable = true;
    description = "Gnome Polkit for sudo via GUI";
    wantedBy = [ "graphical-session.target" ];
    serviceConfig = {
      Type = "simple";
      ExecStart = ''${pkgs.polkit_gnome}/libexec/polkit-gnome-authentication-agent-1'';
    };
  };

  # Virtualization with podman (rootless docker)
  virtualisation = {
    podman = {
      enable = true;

      # Create a `docker` alias for podman, to use it as a drop-in replacement
      dockerCompat = true;
    };
  };

  # 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 = "20.09"; # Did you read the comment?
}

and this is my channel

nixos https://nixos.org/channels/nixos-20.09
nixpkgs https://nixos.org/channels/nixpkgs-unstable