How to activate Roughly Enough Items cheating?

Solution 1:

I had a hypothesis that I have tested, and can confirm firsthand that your world unfortunately does not have enable cheats turned on! "Roughly Enough items" respects the cheats option in the world settings, presumably because in the background it is merely executing commands on your behalf. When you click an item, it is likely just executing a give command. Without permission, it may not be able to do this.

To turn singleplayer cheats on, here is an excerpt from: How to turn cheats on or off with credit to user aytimothy and users who revised the question.

On a singleplayer world:

To turn on cheats:

Using the built-in LAN server:

1. Pause the game.
2. Click on Open to LAN.
3. In the options, change Allow Cheats: false to true (click on it).
4. Open the server.

Changing your save file:

1. Close your world.
2. Using NBTEdit, open your save file (%AppData%/.minecraft/saves/<World Name>/save.dat)
3. Change the NBT node AllowCommands from 0 to 1.
4. Save and reopen the world.