The "LinkAssemblies" task failed unexpectedly

Apparently, this is a known issue: Xamarin Forms needs the TargetAndroidVersion to be 21. (In the config GUI, this is entitled "Compile using Android version:")

In my case problem occured becasue of adding Xamarin.ios.dll to Portable Project references. I have removed that reference and it works now

I changed the Linker properties > Linking from Sdk Assemblies Only to Sdk and User Assemblies for the Release configuration.