Where do I get the customer code to send a non-PO invoice?

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

Before you can start transacting with a customer on the SAP Business Network, you must have a trading relationship with that customer or a unique customer code assigned by the customer. Contact your customer to request a customer code or a trading relationship. Be ready to provide your SAP Business Network ID (ANID) number to them.

If you have a trading relationship with a customer but that customer does not appear in list of customers for non-PO invoicing, the customer does not accept non-PO invoices.

You can check which types of invoices your customer accepts by viewing the customer's invoicing rules.

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