This error occurs when a transaction being uploaded has the same reference as one already in the system. Amend the reference and try again.
Check your supplier account to see if the transaction has already been posted.
If the error persists, it may be caused by other data issues. Upload rows one by one or in small batches to identify which row is causing the problem.
Review the source data for anomalies. If a Sub Ledger code is included, ensure all Sub Ledger codes are valid.