Hi Rui Silva,
Are you able to run the getting started sample which is attached at bottom of this user guide?
https://help.syncfusion.com/xamarin/sfchart/getting-started
If yes, could you please check whether you have properly added the Chart reference in your application by referring following guide.
https://help.syncfusion.com/xamarin/sfchart/getting-started#adding-chart-reference
I would recommend you to add the SfChart assemblies as a nuget from nuget.org to avoid the manual errors.
https://www.nuget.org/packages/Syncfusion.Xamarin.SfChart/
Thanks,
Prabakaran R.