Downgrade Project form Oreo(27) to Lollipop(21)

Hi
I've downloaded VSCommunity2017, used the SDK-Manager to download the proper SDK, made a new AndoidApp(Xamarin) or AndoidXamlApp(Xamarin.Forms), when i switch compile target framework, i get ERROR NU1202 Packet Xamarin.Android.Support.V4. 27.0.2 ist nicht mit monoandroid50(MonoAndroid,Version=v5.0) kompatibel. ...
Any Help would be nice.