You put resources "/items", ItemController, except: [:new, :edit]
inside a scope block as mentioned above.
In your case it should be like this:
pipeline :api do
plug :accepts, ["json"]
end
scope "/", AppWeb do
pipe_through :api
resources "/items", ItemController, except: [:new, :edit]
end
To have an idea of what routes being generated of the resources
then type mix phx.routes
in your terminal.