/difficulty not working. same with tab.

I use /difficulty Same With Tab And Not Works (I Used /difficulty peaceful , Language In Portuguese (Because Im From Brazil (I Understand Everything!))And Well Im On HARDCORE! Btw, My Friend Take A Cheat To Allow Cheats On Hardcore,(Simple, Just Do LAN World And Allow Cheats. (LOL I Have Wroten This As Public))So I Used All Difficultys And Not Worked. Please Anyone Reply! (And I Put None As Game Mode So Why Ill Tested On The 2 Game Modes.)