Negative Acknowledgments, known as NACKs, let the sending system know that there was an error while processing the message.

If there were problems processing the message, a negative ACK message is sent to the sending system. That message will be delivered back into SWTIN folder in DOKA and can be viewed in INFACK .

NACK Messages can be re-processed when the transaction has completed the work flow or not 

Key steps to troubleshoot the NACK :

1.Check for the Rejection reason. It could be due to invalid BIC ,invalid address. Rejection reason can be checked in the message populated in the SWTIN folder or can be checked  in INFACK using the message unique Reference (MUR). 

The highlighted field represents the MUR -Message user Reference which can be used in screen INFACK To query ACK/NACK Records .

2.Rectify the Issue and reprocess the swift in DBXTRN if its final else use DBWTRN  - Transaction to repair workflows ,if the workflow is not completed.

3.If there are no errors in the SWIFT Message, ACK message will be regenerated with a different MUR and can be checked in INFACK or the respective transaction.

Rejection Reason for NACK message :

In the information transaction for electronic messages ’Info Acknowledgement Entry
(INFACK)’ the reason code is  shown. So, there is no need to investigate for the reason
outside of DOKA-NG.