If you're using the UI for Xamarin Nuget package.it adds all the UI for Xamarin references to your project.
This includes the following references to be able to use the new RadRadialGauge control:
As of the latest release (R1 2017), we've added a new component RadRadialGauge. RadRadialGauge is rendered via the SkiaSharp graphics library so you need to install SkiaSharp.Views.Forms.
See my attached screenshot for clarification, I also attached a demo app to compare against yours.
After you add the SkiaSharp Nuget Package to all of the projects in your solution, the error should go away. Let us know if you have any further trouble.
No plans for using RadGauge?
If you do not intend on using the RadRadialGauge, you can remove the two references from each project:
Note that the next time you install the UI for Xamarin nuget package (i.e. for an update), you may have to remove them again as this is part of the install script.