Inspection data button not active in the Inpection Processing Entry form

We are trying to understand what makes the “Inspection Data…” button to become active (is currently greyed out) in the Inspection Processing Entry form. Some of our purchase orders make the button active and others do not. Where is the set-up to control which purchase orders require the “Inspection Data…” button to become active?

Per Application Help:

Inspection Data…

Use this button to start the Inspection Results Entry process. On return the NCM is updated with the passed quantity and the failed quantity.This button is disabled if the part does not have an inspection plan and specification defined.

Thank you for your reply.
I checked the part and it has an inspection plan and specification defined but the spec is set to zero quantity. Could that be causing it to be disabled?