ACH R91- What is this?

ACH code R91 is an Automated Clearing House (ACH) return code that indicates an invalid trace number. A trace number is a unique number that identifies each ACH transaction.
An R91 return code means that the trace number provided in the transaction is invalid or does not match the trace number of the original transaction. This can occur if the trace number was entered incorrectly or the transaction was processed more than once with different trace numbers.
If you receive an R91 code, you should review the original transaction to ensure the trace number was correctly entered. If you cannot identify the issue or correct the error, you may need to contact your bank or the ACH operator for further assistance.