Xamarin.Forms Android Error - new Httpclient()

Question

This error (image) appears whenever you arrive on this line:

using (var client = new HttpClient())

This is the complete code.

using Newtonsoft.Json;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;

namespace LivrosApp.ApiLivros
{
    public static class Api
    {
        public static async Task<IList<Model.Livro>> GetAsync()
        {
            using (var client = new HttpClient())
            {
                var json = await client.GetStringAsync("http://mvalivros.azurewebsites.net/api/livros");
                var livros = JsonConvert.DeserializeObject<List<Model.Livro>>(json);
                return livros;
            }
        }
    }
}

I have remade the solution, I already gave Clean in the solution and in the projects, I built, I rebuilt, activated all the exceptions... Nothing works...

I marked all functions below also.