How do I get the customer code to send a non-Purchase Order invoice?

Why am I receiving the error !Customer code does not match any customer when creating a non-PO invoice?

To create non-PO invoices for a buying organization, you must have either a trading relationship established with your customer or a unique customer code provided by them.

If you are prompted to enter a customer code when creating a non-PO invoice, it means you do not have a trading relationship established with that customer in your account.

Contact your customer and request that they provide you with their customer code or send a trading relationship request to your account. Provide them with your SAP Business Network ID (ANID) when doing so.

If your customer provide you their customer code, once you submit the non-PO invoice and they approve it, the trading relationship will automatically establish in your account.

You can share with your customer the following documentation on how to generate a unique customer code: Receiving Non-PO Invoices from Unregistered Suppliers

If you have a trading relationship established with your customer, but their company name does not appear in the drop-down list of customers when creating a non-PO invoice, it means the customer does not accept non-PO invoices. In the customer's transaction rules, the following rule is disabled (set to No): Allow suppliers to send non-PO invoices.

SAP Business Network for Procurement & Supply Chain > Transaction Documents > Transaction (Invoicing)