Users are not able to post a response to a ticket as there is no “Leave a Reply” section of textbox available.

If a ticket is still open or need customers to submit their comments, you must enable comments in WP Ticket by checking Allow Comments per ticket under Discussion box. If the Discussion box is not visible, check Screen options tab (on the top right of your edit screen) if it is enabled there. You can also enable comments for all new tickets by Settings > Discussion > Check Allow people to post comments on new articles.