Flexible numbering routine returned an undefined number range interval.

What causes this issue?

The billing type of the billing document that you are trying to create
uses a custom routine to determine an appropriate number range interval.
This custom routine is implemented through the BAdI <ZK>Flexible Billing
Document Numbering </>(SD_BIL_FLEX_NUMBERING).
The custom routine that is assigned to the type of billing document that
you are trying to create refers to a number range interval that has not
been defined and therefore does not exist in the system.

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

System Response

No billing documents are created.

How to fix this error?

You can do one of the following:
Correct the custom logic in the BAdI implementation so that it refers to
a valid internal number range interval.
Create the internal number range interval that the custom logic refers
to.
For more information, key users can view the documentation for the BAdI
<ZK>Flexible Billing Document Numbering </>(SD_BIL_FLEX_NUMBERING) in
the <ZK>Custom Fields and Logic </>app.

Error message extract from SAP system. Copyright SAP SE.