Problem while add Xamrine.Android.Support.Design in Xamrine project(visual studio) by using nugget

I am trying to add nugget packages in my Xamarin.Forms project and Xamarin.Android project. I am able to add below list of nuget packages to my Android project but I am not able to add them to the Xamarin.forms project. I checked the target framework in the Android project (right click on android project -> Properties -> Application - > Compile using Android version). It is set to Android9.0(Pie)

Nuggets - 
Xamrin.Android.Support.Design
Xamrine.Android.Support.Core.UI etc

whenever I try to add these nuggets it show below error.

Error   NU1202  Package Xamarin.Android.Support.Design 28.0.0.3 is not compatible with netstandard2.1 (.NETStandard,Version=v2.1). Package Xamarin.Android.Support.Design 28.0.0.3 supports:
  - monoandroid60 (MonoAndroid,Version=v6.0)
  - monoandroid70 (MonoAndroid,Version=v7.0)
  - monoandroid71 (MonoAndroid,Version=v7.1)
  - monoandroid80 (MonoAndroid,Version=v8.0)
  - monoandroid81 (MonoAndroid,Version=v8.1)
  - monoandroid90 (MonoAndroid,Version=v9.0)

For whatever reason, when I am trying to use below in my App.xaml.cs

#if __IOS__
using AVFoundation;
using FM.LiveSwitch.Cocoa;
using UIKit;
#else
using Android.Views;
using Android.Widget;
#endif

it shows the error

Error   CS0246  The type or namespace name 'Android' could not be found (are you missing a using directive or an assembly reference?)