Model service specification & does not exist

What causes this issue?

The <DS:GLOS.model_service_specifications>model service
specification</> you entered has not yet been created.

System Response

The system checks whether the model service specification you enter
exists. If it does not, you see an error message.

How to fix this error?

Create the model service specification first.

Error message extract from SAP system. Copyright SAP SE.