When we POST an invoice or a creditMemo to NetSuite, each line item has values for the custom transaction line fields. The NetSuite API accepts the JSON data, but the field validation returns a warning. Unknown field name custcol_taxcode. The field does not exist on this record instance. Why is that?

That warning usually means the field custcol_taxcode isn’t exposed on the record type you’re posting to. Custom transaction line fields can be scoped e.g. only to certain transaction types, forms, or line sublists. Double-check that the field is applied to invoices/credit memos and specifically to the item sublist. If it’s scoped only to sales orders or another sublist, the API won’t recognize it even though the JSON goes through.

all good. The developer was using the other sandbox where the field doesn't exist. Thanks anw