mod version: 4.2.6
game version: 1.20.1
in data/gateways/gateways/emerald_grove.json
the "stack" type's reward use "id" as item's name
but mod's data is "item" as item's name
maybe my english is terrible.
forgive me about that thanks. =w=

It seems like you were looking at the 1.21 version of Placebo's schema files. In 1.20 the key is "item" (ref: https://github.com/Shadows-of-Fire/Placebo/blob/1.20/schema/ItemStack.md)