ACH Code R81 - What is this?

ACH code R81 indicates an invalid trace number in an Automated Clearing House (ACH) transaction.
The trace number is a unique identifier assigned to each ACH transaction to enable tracking of the transaction through the banking system. The trace number is included in the ACH record as a part of the transaction information, and it must be unique for each transaction.
If the trace number is invalid or duplicates one already used for another transaction, the ACH transaction will be rejected, and the R81 return code will be issued.
The originator of the transaction must correct the trace number and resubmit the ACH transaction for processing.