
"The installed Android SDK is too old. Version 25.1.3 or newer is required. Please update to the latest version."
Trata-se de algum erro no Xamarin. Para resolve-lo, siga os passos a seguir.
1. Abra o Android SDK Manager (No Visual Studio: Tools > Android > Android SDK Manager)
2. Faça o download do SDK Platform do Android 6.0 (API 23).
3. Reabra seu projeto.