You realize that when it is written :l <nixpkgs>, you actually need to type the <nixpkgs> verbatim? This is not a placeholder of any kind, it will be replaced with the path to your channel called nixpkgs automatically.
Additionnally, you can directly type it in the command line, like:
$ nix repl '<nixpkgs>'
> pkgs.linuxPackages.#type TAB here