Cannot install gopls using mason.nvim

Don’t use use the tools from mason. Instead, you should add packages to neovim in home-manager like so:

# home.nix
programs.neovim = {
  enable = true;
  extraPackages = with pkgs; [
    # Formatters
    gofumpt
    goimports-reviser
    golines

    # LSP
    gopls

    # Tools
    go
    gcc
    ...
  ];
};

If you don’t have home-manager installed, you can install it using the following manual:

https://nix-community.github.io/home-manager/

For more inspiration, you can also have a look at this configuration example that someone posted here before: