No functional location exists as reference object

What causes this issue?

The system attempted to generate an IDoc with IDoc type ISU_DL_ORDER,
however, this IDoc type requires a service order with a functional
location as its reference object. The specified order does not have a
functional location as its reference object.

System Response

IDoc generation is terminated.

How to fix this error?

Specify an order that has a functional location as its reference
object.

Error message extract from SAP system. Copyright SAP SE.