Input of document number is only possible with external number assignment

What causes this issue?

The indicator for document number required entry was set, but either an
internal number assignment or no number assignment is preset for this
output type.
The SAP system automatically assigns consecutive numbers during internal
number assignment. It is not possible to make an entry externally. This
indicator is only suitable for external number assignment.

System Response

The system responds with an error message.

How to fix this error?

Deactivate the indicator for the required entry of the document number
or choose an external number assignment.

Error message extract from SAP system. Copyright SAP SE.