The matchsettings (also called gamesettings or playlist file) file you specified in the command line is wrong.
If you save your game settings in UserData/Maps/MatchSettings/royal.txt, your command line should contain /game_settings=MatchSettings\royal.txt.
You can also check for the file permission if you are using Linux.
Your MatchSettings file is specifying an unknown mode, change it to the value 0:
<playlist>
<gameinfos>
<game_mode>0</game_mode>
...Happens on servers which are online for more then 30 days. Restart your server to remove this message for players.