Order unit missing in billing document item &

What causes this issue?

The system has checked whether a billing document can be created with
respect to the income attributable to the condition record. A
precondition for this is the specification of a sales unit. In the case
of a billing document for end-of-period settlement with respect to
rebate arrangements within the Purchasing area, this is the order unit.
However, the system was unable to determine a valid order unit for the
condition record.
The item number in the billing document is &V1&. Please note that in
the case of a billing document at plant level, only one totals line is
outputted in the settlement reconciliation list under the item number
in accordance with the first plant. You will find the relevant material
in this totals line.

System Response

The settlement accounting for this arrangement is faulty; if you wanted
to carry it out for real (instead of a simulation), no billing document
would be created.

How to fix this error?

Please rectify the source of the error and re-run the settlement
program for the arrangement. If necessary, contact your SAP system
administrator.

Error message extract from SAP system. Copyright SAP SE.