I do have an issue with mentioned function. This is the error from browser:

function Bamboo.SentEmailViewerPlug.init/1 is undefined (module Bamboo.SentEmailViewerPlug is not available)

And this is from console :

[error] #PID<0.868.0> running RewardappWeb.Endpoint (connection #PID<0.829.0>, stream id 4) terminated
Server: localhost:4000 (http)
Request: GET /mailbox
** (exit) an exception was raised:
    ** (UndefinedFunctionError) function Bamboo.SentEmailViewerPlug.init/1 is undefined (module Bamboo.SentEmailViewerPlug is not available)
        Bamboo.SentEmailViewerPlug.init([])
        (phoenix 1.6.6) lib/phoenix/router/route.ex:41: Phoenix.Router.Route.call/2
        (phoenix 1.6.6) lib/phoenix/router.ex:355: Phoenix.Router.__call__/2
        (rewardapp 0.1.0) lib/rewardapp_web/endpoint.ex:1: RewardappWeb.Endpoint.plug_builder_call/2
        (rewardapp 0.1.0) lib/plug/debugger.ex:136: RewardappWeb.Endpoint."call (overridable 3)"/2
        (rewardapp 0.1.0) lib/rewardapp_web/endpoint.ex:1: RewardappWeb.Endpoint.call/2
        (phoenix 1.6.6) lib/phoenix/endpoint/cowboy2_handler.ex:54: Phoenix.Endpoint.Cowboy2Handler.init/4
        (cowboy 2.9.0) /Users/mateuszosinski/Desktop/elixir/rewardapp/deps/cowboy/src/cowboy_handler.erl:37: :cowboy_handler.execute/2
        (cowboy 2.9.0) /Users/mateuszosinski/Desktop/elixir/rewardapp/deps/cowboy/src/cowboy_stream_h.erl:306: :cowboy_stream_h.execute/3
        (cowboy 2.9.0) /Users/mateuszosinski/Desktop/elixir/rewardapp/deps/cowboy/src/cowboy_stream_h.erl:295: :cowboy_stream_h.request_process/3
        (stdlib 3.17.1) proc_lib.erl:226: :proc_lib.init_p_do_apply/3

Here is my router.ex file :

defmodule RewardappWeb.Router do
  use RewardappWeb, :router
  use Phoenix.Router

  pipeline :browser do
    plug :accepts, ["html"]
    plug :fetch_session
    plug :fetch_live_flash
    plug :put_root_layout, {RewardappWeb.LayoutView, :root}
    plug :protect_from_forgery
    plug :put_secure_browser_headers
  end

  pipeline :api do
    plug :accepts, ["json"]
  end

  scope "/", RewardappWeb do
    pipe_through :browser


    #get "/", PageController, :index
    #NEW ROUTES
    #get "/users", GrantController, :login
    get "/admin", GrantController, :admin
    get "/main", GrantController, :main
    get "/", GrantController, :index
    post "/", GrantController, :login
    get "/add", GrantController, :add
    post "/add/:id", GrantController, :update
    get "/admin/delete/:id", GrantController, :delete

  end

  if Mix.env == :dev do
    forward "/mailbox", Bamboo.SentEmailViewerPlug
  end

  # Other scopes may use custom stacks.
  # scope "/api", RewardappWeb do
  #   pipe_through :api
  # end

  # Enables LiveDashboard only for development
  #
  # If you want to use the LiveDashboard in production, you should put
  # it behind authentication and allow only admins to access it.
  # If your application does not have an admins-only section yet,
  # you can use Plug.BasicAuth to set up some basic authentication
  # as long as you are also using SSL (which you should anyway).
  if Mix.env() in [:dev, :test] do
    import Phoenix.LiveDashboard.Router

    scope "/" do
      pipe_through :browser

      live_dashboard "/dashboard", metrics: RewardappWeb.Telemetry
    end
  end

  # Enables the Swoosh mailbox preview in development.
  #
  # Note that preview only shows emails that were sent by the same
  # node running the Phoenix server.
  if Mix.env() == :dev do
    scope "/dev" do
      pipe_through :browser

      #forward "/mailbox", Plug.Swoosh.MailboxPreview
    end
  end
end

Mails are being sent totally fine - I have inspected that with IO.inspect, and right now, they are sent fine. However, I can not attempt to see localhost:4000/mailbox. Where is my mistake? Thank you in advance!!