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You may have jumped right to the end and forgot to look at the middleman in this case, category.
When you see to validations like that, its usually due to multi level validation constraints. In this case, it's Service >> Category >> Sub-Category.
If you examine the picklist for category, you will see it's actually using a relationship for the constraint.
Going to category, and relationships, you can see Rev2, linked to Ci.Service
If you find that relationship in the layout of CI.Service, you can see it is a tab in Ci.Service.
So if you want to edit/add categories for a Service, it's done via the 'Service Desk Category' tab on Ci.Service.