Hi Fahad,
We are able to reproduce the reported issue and the reported error will occur due to the missing reference of ‘SyncfusionSfShared.WinRT’ dll.
Actually SfRadialMenuItem is derived from HeaderedItemsControl which is defined in Syncfusion.SfShared.WinRT source hence accessing Header property of HeaderedItemsControl without shared reference results in reported error.
Hence we recommend to add 'Syncfusion.SfShared.WinRT' reference to your app to resolve the reported issue.
Please let us know if you have any queries.
Regards,
Vignesh V