'Invalid captch code'
Please try to deactivate all other CAPTCHA plugins you have, then delete the cache. I think it conflicts with other plugin. If it doesn't help please send admin login details to support[at]gvectors.com email address. We'll login and try to fix all conflicts with your WP plugins and theme.
Also I don't see the reason to purchase an addon if you see the free core wpDiscuz has site specific issue and doesn't work. Please note, our support is ready to help with any issue related to wpDiscuz but it may take 24-36 hours to get answer on your question. Your many will always be sent back if the issue is not solved, don't worry about that.