While someone may be able to help you with this issue, it is unrelated to Kunena. It is a configuration problem with your site setup. Please contact your hosts or look into the documentation for whichever system you site is hosted on.
Kunena does not handle user registration, it is handled by Joomla.
This question has been asked before and, at this stage, there's not much help that I can suggest. I'm sure you didn't intend it in your question, but to clarify a couple of issues for a few of our newer users, Kunena isn't responsible for user registration or login. In this sense, perhaps the following statement might help point you in the right direction:
Kunena is not responsible for user registration, pure and simple. User registration is a Joomla function that may, or may not, be assisted by other components such as JomSocial or Community Builder or others. Kunena makes it possible to utilise these other components.
Perhaps, one day, Kunena will have its own user registration/profiling/social networking features but these do not exist at the moment in the form that many users seem to want. Does that help you?
Kunena does not perform any of the user registration tasks like adding, changing or deleting user accounts. These functions are handled by Joomla. Other third-party extensions (like Community Builder and JomSocial) can assist these account maintenance functions (if you have them installed).
The KunenaUser Administration function allows you to administer certain functions that registered users have in your forum (such as forum "rank" or moderator privileges). The Kunena user table obtains these records from your Joomla user table. In other words, when a new user registers with your site a a Kunena record is created from their Joomla account. When a user is unregistered (i.e. deleted from the Joomla table) the corresponding record is also deleted from the Kunena table.
Sometimes the Kunena table gets out-of-step with the Joomla table. To work around this problem there is a Synchronize Users function that you can use to put the Kunena table back in step, to delete Kunena records where there is no matching Joomla record or add a new Kunena record if a new Joomla record is found.
I hope this information helps you. In future, would you please use a better topic title so that you (and others who have the same question like it) can find it through the forum search. Perhaps you might edit your original message and change the title like I have done in my reply?