错误:找不到类型或命名空间名称'Android‘(是否缺少CS0246指令或程序集引用？)

Install-Package Xamarin.Android -Version 10.0.0.18

using Android.App;
using Android.Widget;
using Android.OS;

<ItemGroup>
  <PackageReference Include="Xamarin.Android" Version="10.0.0.18" />
</ItemGroup>

using System;
using Android.App;
using Android.Widget;
using Android.OS;

namespace HelloWorld
{
    [Activity(Label = "HelloWorld", MainLauncher = true)]
    public class MainActivity : Activity
    {
        protected override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);

            // Set our view from the "HelloWorld" layout resource
            SetContentView(Resource.Layout.HelloWorld);

            // Get a reference to the TextView on the UI
            TextView helloTextView = FindViewById<TextView>(Resource.Id.hello_textview);

            // Write the message
            helloTextView.Text = "Hello World!";
        }
    }
}