Reference to type 'Assembly' claims it is defined in 'System.Runtime', but it could not be found

This happened to me recently. It was from creating an Xamarin application with PCL and changing the target from .NET standard 2.0 to .NET Standard 1.4. After downgrading the Xamarin.Forms from version 3.0.0.446417 to version 2.5.1.527436 in all the projects, it compiled without errors.

Explanation (per request): If you are Targeting before Fall Creators Update (version 6.332), you will need to remove all references to .NET Standard 2.0.

Xamarin Forms 3.x uses .NET standard 2.0. Since you are targeting below Fall Creators Update (version 6.332) the PCL will need to be .NET Standard 1.x.

More information here.