After upgrading my Xamarin Forms Android SDK from 25 to 26 (Android 8.0 Oreo) I got the following error message:
error APT0000: 1: error: Error: No resource found that matches the given name: attr ‘android:keyboardNavigationCluster’.
To fix it, check whether you have the Android 26 SDK installed. In Visual Studio for Mac, go to Tools -> SDK Manager. There you’ll find the list with all installed SDKs. Make sure that SDK 26 is installed.