You've installed Webmin, you can log in and out as root, but you don't want to do everything as root. It's better for both security and convenience if you can log into Webmin as your non-privilged Linux user. The problem: even though you've already got an existing Linux user on the server other than root, that user doesn't show in Webmin. Don't worry, this is completely normal for a new Webmin install.
Here is an illustrated tutorial for how to allow an existing Linux user to login to Webmin.Read more