Make sure that the user account that the game is running under has write access to <gmod>\data\ulib\users.txt
Those commands must be run from the server, so it would be "rcon lua_run blah", unless you can see the server console.
Until you get things sorted out, I strongly recommend finding a way to view the server console, not using rcon/your client console.
You may need help from your hosting company on how to do that.