Here is the error being written
/Library/Frameworks/Mono.framework/Versions/5.16.0/lib/mono/xbuild/Microsoft/NuGet/Microsoft.NuGet.targets(5,5): Error: Your project is not referencing the "MonoAndroid,Version=v9.0" framework. Add a reference to "MonoAndroid,Version=v9.0" in the "frameworks" section of your project.json, and then re-run NuGet restore. (HTherapy)