I have already sent a trading relationship request to my supplier, and they accepted it already. We have even transacted in SAP Business Network in the past.

Why did a document sent to my supplier fail now with the error No relationship with supplier?

Your supplier rejected the trading relationship that was previously established with your company. If you would like to continue working with the supplier, you must send a new trading relationship request to their account.

Even if your supplier has accepted your trading relationship request in the past, this relationship needs to be still active if you want to transact with them.

See PO fails to route to the SAP Business Network due to No relationship with Supplier error for more information about this error and additional corrective actions you can take.

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