Can't resolve the reference

i am facing a issues in deploying xamarin ios app from xamarin forms

Error MT2101: Can't resolve the reference 'System.Void Xamarin.Forms.Internals.INameScope::RegisterName(System.String,System.Object,System.Xml.IXmlLineInfo)', referenced from the method 'System.Void AsNum.XFControls.Stepper::InitializeComponent()' in 'Xamarin.Forms.Core, Version=2.0.0.0, Culture=neutral, PublicKeyToken=null'. (MT2101) (OrbisHealthPatient.iOS)

please help me almost one day am wasting ,Thanks in advance.