You do not need to specify a server user if you specified a port number greater than 1024 and are not running the server as root (in this case, you do not need to be logged in as root to start the server). If you do not specify a user account, the server runs with the user account specified at server startup. Make sure, you use the correct user account when you start the server.
If you do not know how to create a new user on your system, contact your system administrator or refer to the system documentation.
Even if you start the server as root, you should not run the server as root all the time. You want the server to have restricted access to your system resources and run as a non-privileged user. The user name you enter as the server user should already exist as a normal UNIX/Linux user account. After the server starts, it runs as this user.
To avoid creating a new user account, choose the user nobody or an account used by another HTTP server running on the same host. On some systems, the user nobody can own files but is unable to run programs.