ikun hi, are you speaking of registering a new forum on the main FreeFlarum website (https://freeflarum.com), or registering a second forum account on your existing forum? When you attempted to register, did any error messages appear? If you were trying to register a new forum, could you please share the forum name that you were trying to register? It is possible that you have hit forum registration limit, you have used a reserved name or invalid name, or that the forum already exists.
