Please check whether you have all the needed for RadSegmentedControl assemblies referenced in the corresponding projects (Portable, Android):
SegmentedControl: Required Telerik Assemblies
If this does not help, I'd ask you to send us more details on your app, such as: Xamarin.Forms version, UI for Xamarin version, code snippet showing how the SegmentedControl is defined and anything else that you find relevant.
I'm looking forward to your reply.
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