The below error message will be displayed if you are transferring a record that already exists in the destination system. This happens when an end user has already created that record in the destination before it has been transferred by your Rapidi transfer. In order to remove this error, you would need to exclude it from your transfer by adding a filter. Once you do that, you would need to click on Activate Changes and the transfer will not try to transfer again that record.
The REST service returned a status code that is not 20x - i.e. not OK - Status code received: Document Type='Order',No.='506984' Response body: {"error":{"code":"Internal_EntityWithSameKeyExists","message":"The record in table Sales Header already exists. Identification fields and values: Document Type='Order',No.='506984' CorrelationId: 5919490f-5b4a-4e41-b902-000a6ea49d6b."}}