Looks to me like you need to enable the plugin. Are you sure you followed all the steps correctly to install it?
It’s alot of fun! Here’s a screenshot to get you on your way. Note the is_ticket to enable the ticket and that you need a tags[] for each tag you want to add. Also, note that to assign tickets via the API you need to do a follow-up call which I have not figured out how to do via postman.
I also found this useful when learning how to use the API: How to reverse engineer the Discourse API