Errors adding Xamarin.Forms through NuGet

Hi!

After testing Xamarin.Forms, we've decided to add it to our existing iOS app. We downloaded the library from NuGet.

We just added a test ContentPage and, unfortunately, the app don't compile. We have the following issue:

Reference to type 'System.Object claims it is defined in assembly 'System.Runtime, Version 2.6.7.0, 
Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' but it could not be found.

Any ideas on how to solve this?

Thanks!