Namespace of Android, Xamarin and XamarinForms not found
<p>I cloned a Xamarin.Forms project from GitHub and opened it in Visual Studio. There I get 194 errors, because of namespace/usings that are not found.
These usings are red underlined (all except "using System"): </p>

<ul>
<li>using Foundation</li>
<li>using UIKit</li>
<li>using Android</li>
<li>using Windows</li>
<li>using Xamarin</li>
</ul>

<p>The project should work because my team member who pushed it to git can build it on his pc.
I'm very new in Xamarin.Forms and Visual Studio, so please write a detailed answer. :)
Should I install these namespaces, or why doesn't Visual Studio recognize them?</p>