Oh boy, welcome to the world of 1.12.2 Minecraft Forge, where life was a mess. The following video will help you if you're wanting to run the code .-.
https://www.youtube.com/watch?v=rmWBP5ifDlw
This mostly helps if you're on Windows. I provide no assistance to Linux users.
Use the command pattern to add some feature commands to Minecraft 1.12.2 version