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 https://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.
