error:
       … in the condition of the assert statement
         at «github:NixOS/nixpkgs/716c7a2664ca8325617b8a7fbb609273f2c4cae7?narHash=sha256-zbcZ1dmBTPfJ7Mlqh/yLEPGpgJnwuv4Xr1xucy2WqMA%3D»/lib/customisation.nix:416:13:
          415|           drvPath =
          416|             assert condition;
             |             ^
          417|             drv.drvPath;

       … while evaluating the attribute 'handled'
         at «github:NixOS/nixpkgs/716c7a2664ca8325617b8a7fbb609273f2c4cae7?narHash=sha256-zbcZ1dmBTPfJ7Mlqh/yLEPGpgJnwuv4Xr1xucy2WqMA%3D»/pkgs/stdenv/generic/check-meta.nix:727:9:
          726|         valid = "no";
          727|         handled = handle {
             |         ^
          728|           inherit attrs meta;

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

       error: Refusing to evaluate package 'gitbutler-0.21.0' in /nix/store/h84gf3axlp9jjzdlykzi65qa5j3w7xa8-source/packages/gitbutler/package.nix:151 because it has an unfree license (‘fsl11Mit’)
       a) To temporarily allow unfree packages, you can use an environment variable
          for a single invocation of the nix tools.

            $ export NIXPKGS_ALLOW_UNFREE=1

          Note: When using `nix shell`, `nix build`, `nix develop`, etc with a flake,
                then pass `--impure` in order to allow use of environment variables.

       b) For `nixos-rebuild` you can set
         { nixpkgs.config.allowUnfree = true; }
       in configuration.nix to override this.

       Alternatively you can configure a predicate to allow specific packages:
         { nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
             "gitbutler"
           ];
         }

       c) For `nix-env`, `nix-build`, `nix-shell` or any other Nix command you can add
         { allowUnfree = true; }
       to ~/.config/nixpkgs/config.nix.