Material &1 (qualifier &2) could not be identified

What causes this issue?

The material referred to by the stock or sales data could not be
determined by the system.

System Response

The system cancels inbound processing.

How to fix this error?

The system identifies the material in the IDoc PROACT by means of the
section E1EDP19. You can use both the customer material number
(qualifier '001') and vendor material number (qualifier '002'). If you
choose to use the customer material number, you must maintain it
accordingly in the master data.
Identification of the article using EAN or UPC is not supported.
Configure your EDI converter in such a way that it is possible to
identify the material.

Error message extract from SAP system. Copyright SAP SE.