tetra_parry_datapack.zip
I have no Idea what went wrong with it.

{
  "parryAttributes": "sword",
  "items": [
    "tetra:modular_sword",
    "tetra:modular_shortblade"
  ]
}

tetra:modular_shortblade doesn't exist, there is no such item.
Just remove it and everything will be ok.

And remember that setting parry attributes for items doesn't enable parrying with them (i know it's not intuitive, but it was the easiest and fastest way for me to implement).

To be able to parry with modular sword you have to add it to "items_can_parry" tag (data/frycparry/tags/items/items_can_parry.json)