Looking at the Xamarin.Android.Support.v7.AppCompat 21.0.3.0 NuGet package, it contains only the build for MonoAndroid 4.0.3, so your project will need to target this version or higher before you can install it.
It looks like you are using Visual Studio so that you can either enter the project settings or change the target version of Android in the project properties and change Compile using the Android version or edit the project file (.csproj) and change TargetFrameworkVersion directly in the file.
<TargetFrameworkVersion>v4.3</TargetFrameworkVersion>
If you need to target Android 2.2, you can use the Xamarin.GooglePlayServicesFroyo NuGet package, as it can be installed in a project targeted at MonoAndroid 2.2
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