Category-wise assignment also needs a Default Assignee ID in global settings.
See Constant Editor.
Ticket Categories
Create Helpdesk ticket categories on the storage page (ns_helpdesk 14+).
From ns_helpdesk 14.0.0, ticket categories are no longer managed in the
Helpdesk backend module.
Show or hide the categories field, and set its label, in the Constant Editor.
See Form Settings.
Create or edit category records on the Helpdesk storage page (the Global Storage PID
from global settings):
Step 1. Open the List module.
Step 2. Select the storage page.
Step 3. Create a category record.
Step 4. Set the category title and color, then save.