External document assignment not possible with record type &

What causes this issue?

The record type is linked to a number range group containing no
external number ranges.

System Response

The system requires a document number.

How to fix this error?

Delete the document number or have your system administrator define an
automatic external number assignment for the record type.

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.