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