The solution that worked for me:
Uninstall Xamarin.Forms:
Right Click Solution -> Manage NuGet Packages -> Uninstall Xamarin.Forms -> Restart VS
Then remove build platforms:
Right Click Solution -> Properties -> Build -> under Targeting select Change -> Remove platform(s) -> Restart VS
Reinstall Xamarin.Forms:
Manage NuGet packages -> Search for Xamarin.Forms -> Install -> Restart VS