What causes this issue?

You want to create a default rule for the
<DS:GLOS.settlement_rule>settlement rule</>.
However, this <DS:GLOS.maintenance_order>maintenance order</> or
<DS:GLOS.service_order>service order</> may not be settled.

System Response

INCLUDE KEINE-VERARBEITUNG OBJECT DOKU ID TX LANGUAGE EN

How to fix this error?

In your System, the Customizing function was used to define that
settlement is not intended for orders of the <DS:GLOS.order_type>order
type</> being used by you. A settlement rule is, therefore, not
necessary.

Error message extract from SAP system. Copyright SAP SE.