From: https://developer.apple.com/library/archive/documentation/DataManagement/Conceptual/CloudKitWebServicesReference/QueryingRecords.html#//apple_ref/doc/uid/TP40015240-CH5-SW4 Queries can be configured against system fields as well as developer-defined fields. The system fields are special fields on which an index can be defined. These fields have a special mapping such that the ___ prefix does not need to be used. The sort descriptor dictionary can take a key systemFieldName instead of fieldName where the mapped values can be used. You also need to add a 'Sortable' Index to the Record's CreatedTimestamp Field in CloudKit Console.

Did you ever figure this one out? I'm getting same errors ...