Internal number assignment not possible

What causes this issue?

You want the system to internally assign a number for the group and
have, therefore, not entered a number.
Your system currently does not permit internal number assignment. You
can define internal and external assignment data in Customizing for
routings under <ZK>Control data</>.

How to fix this error?

Assign a number for the group.

Error message extract from SAP system. Copyright SAP SE.