This is the message i receive when I try to enter a command into a command block.
I am making an adventure map - the world was created as "adventure mode" but I am switching to "creative mode" via TooManyItems to build it. Even when I am in this creative mode I still get the message.
You have to have cheats on. Either put it on while you starting a new world or open a LAN and put cheats on. There is also a third party program called gamemode changer that can change already set maps that I used a lot.