External number assignment not possible for down payment clearing doc.

What causes this issue?

You have released a billing document to accounting, and down payment
clearing is being carried out for this billing document in FI. The down
payment to be cleared was posted in a different currency to the billing
document. Down payment clearing takes place in the transaction currency
of the down payment, and therefore two FI documents arise from the
transaction: One document for the customer invoice and one for the down
payment clearing. However, since you have configured external document
number assignment for the number range interval of the FI document to
be created, the system uses document type &2 and external document
number assignment for the customer invoice document, and internal
document number assignment for the down payment clearing document,
together with the standard document type defined with the Customizing
transaction OBU1 for transaction F-39, that is, the standard document
type &1 for down payment clearing. However, internal document number
assignment must be configured in FI for the number range interval for
the down payment clearing document to be created. This is not the case
in your system.

System Response

The accounting documents for the billing document are not created.

How to fix this error?

In this case, there are three possible solutions:
1.,,Configure internal document number assignment for the number range
interval of the FI documents to be created from the ,,billing document.
2.,,The billing document and the down payment use the same transaction
currency.
3.,,Configure internal document number assignment for the number range
interval of the ,,
down payment clearing document to be created in FI.

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

Error message extract from SAP system. Copyright SAP SE.