I believe certain forums are setup so posts are not allowed unless you are now licensed. If you have a license and you cannot post, it may be that you used a different email address during purchase than you did when signing up on xf.com. Simply add your xf.com user name to your license area. It may take a few minutes for that to take effect.
"You must be a XenForo customer to post in these forums. Ensure your account is listed here."