Poolboy example error: (Mix) Could not start application poolboy_py_elixir

(Mix) Could not start application poolboy_py_elixir: PoolboyPyElixir.Application.start(:normal, []) returned an error: shutdown: failed to start child: :square_worker
    ** (EXIT) an exception was raised:
        ** (MatchError) no match of right hand side value:
{:error, {:EXIT, {:undef, [{PoolboyPyElixir.SquareWorker, :start_link, [[]], []}, {:supervisor, :do_start_child_i, 3, [file: 'supervisor.erl', line: 414]}, {:supervisor, :handle_call, 3, [file: 'supervisor.erl', line: 439]}, 
{:gen_server, :try_handle_call, 4, [file: 'gen_server.erl', line: 721]}, 
{:gen_server, :handle_msg, 6, [file: 'gen_server.erl', line: 750]}, 
{:proc_lib, :init_p_do_apply, 3, [file: 'proc_lib.erl', line: 226]}]}}}

The sample repo’s application.ex is here.

I am trying to follow this post.

This is first time trying to understand poolboy. Any help is appreciated.