Since you're new, you don't have reputation points yet to post comments.  When you reach 50 points, you'll be able to do so.  Please see http://askubuntu.com/privileges/comment for more information.

The reason for this is to prevent abuse from spammers and bots.  It won't take long to have 50 points - you already have 30! - and you'll be able to post comments.  By the way, there are a lot of privileges that you obtain over time as you accumulate more reputation points.