Hi Yasmin,
Thank you for using Syncfusion products.
The issue is because you have declared both the controls in the same name “syncfusion”. In the xaml it is not allowed to have the same name for different namespaces. Please make changes as below to solve the reported issue.
xmlns:syncfusionbuttons="clr-namespace:Syncfusion.XForms.Buttons;assembly=Syncfusion.Buttons.XForms"
xmlns:syncfusionpicker="clr-namespace:Syncfusion.SfPicker.XForms;assembly=Syncfusion.SfPicker.Xforms"
Regards,
Paul Anderson