I don't know if it's because I don't add it or not or if it won't include "MaterialMixing" if it already exists in 1.21.1 of neoforge, how do I enable it because it doesn't appear in the old versions folder?

Legacy material mixing was removed. The Super Mixer is the closest thing now. It has no recipe, so you will need to add a recipe yourself. Or just cheat one in, whatever works for you.