To stop players from cheating, we have disabled Creative mode whilst in-game. You can choose to undo this by running the command
/tag @s add adminthen/gamemode creative
If you are in a single-player world, to run this command you need cheats enabled. To make that change. Pause Minecraft, press Open to LAN, change Allow Cheats to be ON then press Start LAN World.
Now you will be able to run the above command.