Could not install package 'Validation 126.96.36.19903'. You are trying to install this package into a project that targets 'Xamarin.iOS,Version=v1.0', 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.
This is because the NuGet dependency behaviour is set to 'lowest' and
Validation 188.8.131.5203 is not compatible with
Here is a workaround:
Validation NuGet package first, using the latest version
Xamarin.Auth, and it should work as expected without the error.