Net price must be greater than 0

What causes this issue?

The net price is less than or equal to 0, but the invoice receipt (IR)
indicator has been set.

How to fix this error?

If you want to create a <GL:Bestellung>purchase order</> without a
value, you must set the IR indicator to " ".
IF &[SWITCH]MM_SFWS_DINV_01& = 'X'.
If you are seeing this message in differential invoicing, check the
conditions of the conditions of the predecessor document (purchase order
or goods receipt) and correct them, so that invoicing is possible on the
invoice date.
ENDIF.

Error message extract from SAP system. Copyright SAP SE.