title: Visual Studio can't update packages
body: <p>Today, I create a new <code>Xamarin.Forms</code> project, in this project, I want to update the packages, see the picture:</p>
  
  <p><a href="https://i.stack.imgur.com/pV2Dj.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/pV2Dj.png" alt="enter image description here"></a></p>
  
  <p>But, I got this error:</p>
  
  <blockquote>
    <p>Package restore failed. Rolling back package changes for 'App37.Android'.</p>
  </blockquote>
  
  <p>Or you can refer to this picture:</p>
  
  <p><a href="https://i.stack.imgur.com/3sJtk.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/3sJtk.png" alt="enter image description here"></a>             </p>
  
  <p>You also can see many warnings, but I don't care about them. I am only focused on the error.</p>