Quantity entered smaller than quantity still to be invoiced

What causes this issue?

You entered a smaller quantity than the quantity for which delivery
costs are still to be calculated.
INCLUDE MM_IV_MELDUNGEN OBJECT DOKU ID TX
Check your entry and correct it if necessary. If it is incorrect,
select <LS>Cancel</> and call up the function <LS>Delivery costs</> -->
<LS>Select</> again to obtain the correct default values.

Error message extract from SAP system. Copyright SAP SE.