You cannot process conditions for a subsequent debit/credit

What causes this issue?

If an item is subsequently debited or credited, you cannot change the
item conditions.
You can only display the original item conditions. You must enter the
subsequent debit/credit manually.

System Response

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

How to fix this error?

Enter the amount of the subsequent debit/credit and the quantity to
which this amount refers. You enter this manually on the item screen.

Error message extract from SAP system. Copyright SAP SE.