Why are you only allowed to comment with 50 rep?
When I am in 'AskUbuntu' look an Ask I can answer but I can't comment. And I can see other people that can comment it.
I have 6 of reputation and 3 badges.
That is ok? Why?
This is not a bug; this is the intended behavior of AskUbuntu.
You can always comment on your own question or answer (that way, you can always reply to comments directed at you).
To be able to comment everywhere, you must have at least 50 reputation. Which badges you've earned, or how many, is not taken into account for the commenting privilege (nor for any other privilege).
This question explains the reason for this policy. In summary, it's because as a question-and-answer site, we want to focus on asking and answering questions. Comments are secondary. (Unfortunately, many users with over 50 reputation--myself included, sometimes--post unnecessary comments...but that's a different topic. ;-))
It is normal that a user with less than 50 reputation cannot comment on any post, there is a reason for that.
Answers / questions created by new users will show up in tools available to higher reputation users and to moderators, makes it easy to review them and catch spammers, edit the posts to help out the user and review those posts for faults.
Comments show no where in those tools so they are not reviewed / moderated.
This means that in no time we would have the site filled with comments advertizing from Rolex watches to sexual enhancement pills... You get the idea.
The site is built so that if you are a new user you should gain some reputation by making and answering questions and once you have gotten some ground (50 reputation is nothing) you will gain the privilege of commenting. Its just a (fair imo) safe guard to spam and nasty comments that could be created by someone with no reputation.