Why do I see a Purchase order has already been fulfilled message on my order?

Why is there a message saying This is QN legacy purchase order at the top of my PO?

These messages only display for legacy purchase orders.

Legacy orders are sent to your account after their fulfillment and are used for the purposes of invoicing only. You don’t need to worry about delivering items or services from the line items. Instead just submit invoices through SAP Ariba for this type of order.

Regardless of the routing method you have configured for purchase order notifications, you can only view legacy orders in the Workbench of your Business Network account. The Business Network routes legacy purchase orders to the Workbench, even if you use a routing method other than Online. See Not receiving legacy purchase orders via Email, cXML, or EDI routing for additional information about this topic.

SAP Business Network for Procurement & Supply Chain > Transaction Documents
SAP Business Network for Procurement & Supply Chain > Transaction Documents > Transaction (Purchase Orders, Ship Notices & Goods Receipts)