Hi Henry,
Greetings from Syncfusion.
We suspect that the problem is due to the Syncfusion.Xamarin.SfChart packages is not installed properly in all projects . Can you please crosscheck by whether the NuGet package is installed in android and iOS project by referring to the screenshot below?
And ensure whether you have initialized the charts in your application by referring the below steps for iOS and UWP:
Also refer the attached sample for your reference.
If you still face the problem please revert us by modifying the sample, it will be helpful for us to provide you a better solution at the earliest.
Regards,
Rachel.