@Jaymin is right. You must make sure that the version of the installed Xamarin.Android should be as same as the version that you have added to the current project.
For example, if you have installed v27.0.2.1 of Xamarin.Android, you can not upgrade your project to v28.0.0.1 only. You should upgrade the component(Xamarin.Android) which is installed with Visual Studio first.
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