Hi Amrita,
System.Data.DataSetExtensions is not a namespace, but an assembly - methods from it are added to namespace System.Data.
Might be a coincidence, but when manually typing
DataTable newTable = myQuery.CopyToDataTable
(where myQuery is IEnumerable)
even though Intellisense doesn’t work for it, it added a reference to DataSetExtensions and the workflow works just fine.
You could check manually in a text editor if the xaml has the assembly reference to System.Data.DataSetExtensions.dll and if not, add it directly there.
Regards.