trying to make a simple datapack but idk what I am doing and everytime I try it refuses to work.

I am just trying to make a simple "superflat" world for version 1.21.5 with 1 bedrock layer, 120 layers of stone, 50 layers of dirt and one layer of grass blocks. I would like to have the biome be old growth taiga including the trees.
I try to use the generator, https://misode.github.io/ but im not sure if im just messing this up or it just wont work. I used a premade datapack and I try to modify it, but it dosent work either. attached is my file structure. im just kinda annoyed that the command for preset "superflat" worlds known as "decoration" is no longer used, if that was not the case I probably would not have this problem and need to make a datapack. I really tried to use the wiki for any information, but it seems I am making a major misstep (or a few) somewhere. any help is greatly appreciated!!