Check out the newer post on Xamarin.Forms: https://blog.xamarin.com/making-ios-11-even-easier-xamarin-forms/
That warning is because you probably pressed the green Run with Debugging button. Instead, just use the "Project -> Build" option instead of trying to run it.
That's usually an indication that the NuGet package was not installed into the platform (Android, iOS) projects. Try adding the package to to those projects too.
October 11, 2017 - I just posted Updating Xamarin.iOS Apps for the iPhone X!
Read the blog post here
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I'm not 100%, but this might be because of a missing flag for C++. Try adding