How do i solve this issue: Missin directive using...

I have this:
[Activity(Label = "DatePickerService", Icon = "@drawable/calen", Theme = "@style/MainTheme", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]

Label, Theme, MainLauncher and more have problem(CS0246 error).

How can i solve this?

below code

using System;

using Android.App;
using Android.Content.PM;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Android.OS;
using Xamarin.Forms;

namespace DatePickerService.Droid {
  [Activity(Label = "DatePickerService", Icon = "@drawable/calen", Theme = "@style/MainTheme", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
  public class MainActivity: global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity {
    protected override void OnCreate(Bundle bundle) {
      TabLayoutResource = Resource.Layout.Tabbar;
      ToolbarResource = Resource.Layout.Toolbar;

      base.OnCreate(bundle);

      global::Xamarin.Forms.Forms.Init(this, bundle);
      LoadApplication(new App());
    }
  }
}