As described in the Wiki Help article, the Wiki does not allow underscores in user names. The RR Wiki user database is shared with the site user database, but the Wiki is the only application on the site that has the underscore problem. Once a site username is created, it becomes part of the Wiki user database, but underscores are not allowed. It's a bit of a Catch 22.
Your only option is to have your site user name changed.