VF01 creates a billing document with a reference to the eligible sales document. This course uses delivery-related billing, so the delivery must already have goods issue. The order's billing block must also be clear. A delivered shipment can therefore remain unbilled until the commercial review is complete.
Billing posts revenue and a customer receivable at the selling price. The earlier goods issue posted inventory and cost of goods sold at the material valuation price. Don't confuse those two amounts or enter the delivery twice. The training client rejects duplicate billing for the same delivery. Your check needs both a billing document with the right amount and a document flow that reaches billing from the intended order.