Cannot resolve "Invalid PO number" error in Correct Import Errors spreadsheet (no column for PO #)

Summary:

In the Correct Import Errors spreadsheet, I am getting header-level error "Invalid PO number". But there is no header-level field for PO Number, so I am not sure how I am expected to correct this import error? Reposting this from an SR as I was told that asking how to resolve this error is a "how to" question. Any help would be greatly appreciated.

Content (please ensure you mask any confidential information):

After running the Import Payables Invoice process (with source = B2B XML invoice) to import B2B invoices from the Collaboration Messaging module, some invoices are getting import error of "Invalid PO number" for the invoice header.