error: 'builtins.storePath' is not allowed in pure evaluation mode

[ChocolateLoverRaj]

I am new to NixOS. I watched https://youtu.be/a67Sv4Mbxmc?t=516 and am getting an error:

[rajas@nixos:/etc/nixos]$ sudo nixos-rebuild switch --flake /etc/nixos/#default
warning: creating lock file '/etc/nixos/flake.lock'
building the system configuration...
trace: warning: The option `services.xserver.xkbVariant' defined in `/nix/store/d8iv8xbhh7bxqw9f02b80xn1xkqfff6n-source/configuration.nix' has been renamed to `services.xserver.xkb.variant'.
trace: warning: The option `services.xserver.layout' defined in `/nix/store/d8iv8xbhh7bxqw9f02b80xn1xkqfff6n-source/configuration.nix' has been renamed to `services.xserver.xkb.layout'.
error:
       … while calling the 'head' builtin

         at /nix/store/y45vqv6pa8bhgag1dw86rvi6rk55xhxn-source/lib/attrsets.nix:1541:11:

         1540|         || pred here (elemAt values 1) (head values) then
         1541|           head values
             |           ^
         1542|         else

       … while evaluating the attribute 'value'

         at /nix/store/y45vqv6pa8bhgag1dw86rvi6rk55xhxn-source/lib/modules.nix:809:9:

          808|     in warnDeprecation opt //
          809|       { value = builtins.addErrorContext "while evaluating the option `${showOption loc}':" value;
             |         ^
          810|         inherit (res.defsFinal') highestPrio;

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: 'builtins.storePath' is not allowed in pure evaluation mode

       at «none»:0: (source not available)

I am on nixos-unstable. This is /etc/nixos/flake.nix (I didn't modify it):

{
  description = "Nixos config flake";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";

    # home-manager = {
    #   url = "github:nix-community/home-manager";
    #   inputs.nixpkgs.follows = "nixpkgs";
    # };
  };

  outputs = { self, nixpkgs, ... }@inputs: {
    nixosConfigurations.default = nixpkgs.lib.nixosSystem {
      specialArgs = {inherit inputs;};
      modules = [
        ./configuration.nix
        # inputs.home-manager.nixosModules.default
      ];
    };
  };
}