Cash discount basis cannot be calculated correctly

What causes this issue?

You entered an invoice or a credit memo that fulfills all the following
conditions:
Not all items assigned are liable for cash discount
No fixed cash discount amount has been entered
A percentage has been entered for cash discount term 1
A difference exists (manually or automatically accepted)

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

INCLUDE MM_IV_MELDUNGEN OBJECT DOKU ID TX

System Response

The system cannot calculate the correct cash discount basis, since no
information exists on whether the difference is liable for cash
discount and if so how much.

How to fix this error?

In the header enter a fixed cash discount amount to be apportioned
among the items liable for cash discount.
If you switch off the message in Customizing or define it as an
information message or warning message, the system does not take the
difference into account when calculating the cash discount.

Error message extract from SAP system. Copyright SAP SE.