Do not fill the "Document number" field

What causes this issue?

You have entered an external document number. However, the assigned
document number interval is controlled with internal document number
assignment.

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?

Check your entry and, if necessary, change the system settings for the
number range interval.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.