If you don’t see a box in which to post a question for your theme, it probably means that your premium theme wasn’t purchased under your account. Only users who’ve purchased a theme are able to post here.
Try asking the person who purchased your premium theme to log in and post the question here instead, or feel free to post in the free themes forum and we’ll be glad to help you sort it out there:
https://en.forums.wordpress.com/forum/themes