I have someone who wants to register but he gets a message that his username already exists.
I've checked it and there is noone with that name. I;m sure about that cause the forum is new and we are only 10 people registered.
What can i do to help him?

Users & groups>Inactive users
Activate him there.

If that username is not there, do this.
General>Forum>Security
Unauthorize subscriptions made with the most commonly used spam usernames. :No
Save
P.s.: By doing this setting you are opening a gateway for possible bots to register on your forum with most commonly used usernames which may spam.

Problem solved. thx for your help