On Linux you can access NoMachine server settings by entering root credentials or credentials of a user with sudo privileges.
To grant sudo privileges to your user, you must add it to the sudoers group.
Commands for giving sudo privileges to user can be different, so you should check the official documentation for your Linux distribution.
Please note also that in the current implementation it’s not possible to enable changes in server settings GUI on machines with versions of sudo prior to 1.7.4. Authentication as root should work instead.