Prior to Release 4.0, invoices could not be verified for goods receipts for which no purchase order exists. You always had to enter a purchase order number.
As of Release 4.0, you can now configure each movement type in Customizing to allow a purchase order to be automatically created after goods are received. By doing so, you can verify an invoice immediately after the goods receipt is posted. The system valuates the goods receipt at the price defined in the purchasing info record.
You can only generate purchase orders automatically for valuated materials. This function will also be available for non-valuated materials for a later release.
In Customizing, you define for each movement type whether a purchase order is automatically created when a goods receipt is posted, in the step Create purchase order automatically.