Unique assignment of the item using &1 &2 is not possible

What causes this issue?

The IDoc contains an item with reference to <(>&<)>1 <(>&<)>2 from a
transportation management system.
For <(>&<)>1 <(>&<)>2, a number of freight settlement documents were
transferred. A unique assignment of the EDI invoice item to a freight
settlement document is therefore not possible.

System Response

The invoice cannot be posted.

How to fix this error?

For technical reasons, one purchase order is created per freight
settlement document, which the system requires for invoice creation.
In the IDoc, include the number of the purchase order that was created
with reference to this freight settlement document as a selection
criterion in addition to <(>&<)>1 <(>&<)>2.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.