Minecraft game mode changing problems?

Ok, so me and my friend play minecraft on realms and the thing where you type "gamemode c " or s in the talk bar then it changes the mode to survival or creative. Well some reason it doesn't allow me to do that. It just says in red, "you are not permitted to use this command" me or my friend don't know how to change that so could you help please if you know what I'm talking about?
THANK YOU! :)

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