Order item & & will be ignored

What causes this issue?

You have entered an amount, but not a quantity, for purchase order item
&V1& &V2&.
The system will not take into account this item and the amount when the
invoice is posted, which will result in a balance in the invoice.
As a rule, the balance is the same as the amount; however it can differ
if unplanned delivery costs are distributed to the item.
INCLUDE MM_IV_MELDUNGEN OBJECT DOKU ID TX

System Response

The system cannot post the invoice correctly.

How to fix this error?

Please also enter a quantity for this purchase order item.

Error message extract from SAP system. Copyright SAP SE.