Equipment number & not in external number interval

What causes this issue?

The equipment number that you have entered is not in the number
range that has been defined for external equipment numbers.

System Response

INCLUDE 'KEINE-VERARBEITUNG' OBJECT DOKU ID TX LANGUAGE E

How to fix this error?

Enter an equipment number that is in the external number range or use
internal number assignment.

Error message extract from SAP system. Copyright SAP SE.