Invalid toolchain error prevents uploading my Xamarin IOS app

I'm using Visual Studio on a Mac to build my IOS app. I've uninstalled and re-installed XCode and Visual Studio.
When I use Transporter to upload the app, I get the invalid toolchain. Your app was build with an unsupported SDK or version of XCode.

How do I fix this?
Where are the setting to change the tools used to build the app or change the SDK used?

Thanks,
Gerry