I want to ask a question to a user on Ask Ubuntu, but I don't understand how to do it. There is no email, no nothing.
Can you tell me how to do it?
You can only directly contact another user under certain circumstances.
Right now, as you are a 1 rep user, you can't, unless you asked a question or left an answer and someone else commented on it then you can "ping" them with @ followed by their user name. For example to ping me you would write @markkirby in your comment and I would receive a notification about your comment or I would write @dannorris to ping you.
Once you have the privilege to comment anywhere (requires 50 rep) there are a few other ways to make contact using the same @ method above.
These are, for most users, the only way to directly contact someone on the site, please understand, this is not a social network and "chatting" is frowned upon (outside of chat of course), so the ways to contact someone are limited to only post interactions.
Some exceptions do exist, a mod can contact anyone, any time and some users will leave e-mail or social network details in their profiles, so it is worth a look if you really want to contact someone. Please also respect the fact that some users simply do not wish to be contacted off site.
This might seem a bit unfair to a new user but the limitations exist for a reason, these posts may help you to earn the 50 rep: