Xamarin.Forms Android $(TargetFrameworkVersion) error

Posting this as an answer for other people since I almost missed it in the comments. Credit to @FreakyAli

Right-click on Android Project>Properties>Application>Compile using Android version change this to v9 or above and see if that works

On VS Mac it can be found on the General section. (right click android project>Options>General)

I had to manually change the references to the TargetFrameworkVersion in the csproj file to get it to update properly. For some reason there were several references to this value in the .csproj file, all with different values!

I resolved by adding making below settings under Android Project-->Properties and changing target versions