MPN material & not allowed or not found

What causes this issue?

The shipping notification for purchase order no. &V3&, item &V4&, cannot
be adopted.
When determining the MPN material from the MPN and manufacturer shown in
the IDoc, the system did not find MPN material &V1&.
The error may be due to one of the following reasons:

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

The MPN material was not found (value: '_') - i.e. the MPN/manufacturer
combination does not exist in your system.
The MPN material &V1& was found but does not tally with the MPN material
&V2& in the IDoc.
The MPN material &V1& was found but does not tally with the
inventory-managed material of the purchase order.

How to fix this error?

You have the following options:
Adjust your inventory-managed material so that this MPN material is also
allowed.
Change the MPN material in the IDoc.
Get the supplier to send you a new shipping notification with a suitable
material number or suitable combination of MPN and manufacturer.

Error message extract from SAP system. Copyright SAP SE.