Hakan_Sef I was able to register a new account and successfully start a new discussion.
Is the “Start discussion” button visually disabled and can’t be clicked by your users (meaning they don’t have the correct permissios), or do they see a different kind of error message (e. g.: the red “Oops!” frame in bottom left corner of the website) that would indicate an issue with an extension or something else? Do you have any anti-spam extensions enabled that could interfere with your users’ ability to create new discussions? In which tag are they trying to post?