Internal suppliers are not allowed for purchase order generation

What causes this issue?

Supplier &V1& is an internal supplier. You must enter an external
supplier so that you can automatically create a purchase order for
material &V2& in the background.

System Response

You can enter the correct supplier for the goods receipt item by using
the following transaction.
The transaction is started automatically immediately after this
information screen and is supplied with values from the goods receipt
interface.

Error message extract from SAP system. Copyright SAP SE.