I tried to use this command by sitting on the chair with the guide master, and there was no response.
There are two chairs in there only one will work.
Go to "GameServer\Data\GameServerInfo - Event.txt"
and check what settings do you have there on ; King of MU Event Settings.
Basically ReiDoMUMap, ReiDoMUCordX and ReiDoMUCordY is the exact spot where you need to type /rei
Make sure you are exactly on this spot and the event is running obviously.
On my server I do it like this.
