Invalid or inactive category
Create-ticket API returned support.invalidTicketCategory (Invalid or inactive category.) for a bad or disabled category id.
Why this happens
Stale form after admin deactivated a category, or crafted/invalid category id. Distinct from empty category list (no categories available).
Diagnose and fix
1. Refresh the create form; pick a category from the current dropdown.
2. Admin: reactivate or create categories under Ticket categories.
3. Retry create.
4. See Could not create ticket for generic create failures.