This code shows you how to render a phoenix error view inside a Plug

render_phoenix_error_view_inside_plug.ex

defmodule MyAppWeb.Plug.Authorization do
  @behaviour Plug

  import Plug.Conn

  def init(opts), do: opts

  def call(conn, _) do
    with {:ok, token} <- token(conn),
          {:ok, current_user} <- authorize(token) do
      assign(conn, :current_user, current_user)
    else
      _ ->
        conn
        |> put_status(401)
        |> Phoenix.Controller.put_view(MyAppWeb.ErrorView)
        |> Phoenix.Controller.render(:"401", %{})
        |> halt()
    end
  end
end