The line on page 65 (using the PDF) is

 on_mount: [{PentoWeb.UserAuth, :ensure_authenticated}] do

This is incorrect and will throw an error. Searching through the whole codebase shows that ensure_authenticated does not exist. The correct line is

on_mount: [{PentoWeb.UserAuth, :require_authenticated}] do

Full context:

  ## Authentication routes

  scope "/", PentoWeb do
    pipe_through [:browser, :require_authenticated_user]

    live_session :require_authenticated_user,
      root_layout: {PentoWeb.Layouts, :root},
      # Specify the shared on_mount callback here
      on_mount: [{PentoWeb.UserAuth, :require_authenticated}] do
      live "/users/settings", UserLive.Settings, :edit
      live "/users/settings/confirm-email/:token", UserLive.Settings, :confirm_email
      live "/guess", WrongLive
    end

    post "/users/update-password", UserSessionController, :update_password
  end