Xamarin - Visual Studio - Android 尽管已安装但编译版本不存在

Xamarin - Visual Studio - Android compile version not present despite being installed

我有 运行 sdkmanager 命令行和 SDK 管理器,通过 Visual Studio 添加 28,但它仍然不会显示在我的 Visual Studio 属性中。

如果我在 .csproj 中手动更改它,我得到:

Project [PROJECT] cannot build using TargetFrameworkVersion v9.0 because it s not supported. Please change your Xamarin.Android project to target an API version that is supported.

我错过了什么?如何让 Visual Studio/Xamarin 识别它已安装?

please follow this link may it help you 

https://forums.xamarin.com/discussion/129245/project-xyz-droid-cannot-build-using-targetframework-v8-1-because-it-is-not-supported-error