Tried multiple things, can't seem to make it work at all. I attached a .txt version of the Piece Pack, hope that it's a simple thing I'm just not noticing.

grass.txt

You were missing some commas and a curly bracket. Here's a fixed version. Hope this helps!
Note: the grass will probably look grey because of how Minecraft colours the texture based on biome and that not being set up for the pieces. I may do something to help this in the future.
grass.txt