REST Sharp

Hi,
i try Xamarin.Forms for my first project. I have a problem when i try to include RESTSharp.
I get this error: "Could not install package 'RestSharp 104.4.0'. You are trying to install this package into a project that targets 'portable-net45+wp80+win+MonoAndroid10+MonoTouch10', but the package does not contain any assembly references or content files that are compatible with that framework. For more information, contact the package author."

I try to add to Xamarin.Forms packet or Android but the error remain.

Where am I wrong?

Thanks,
Jacopo