Hmm. I checked it out myself and there seems to be some kind of bug there. If you do it how I wrote before, you get the error you describe (and it won’t load). If you use the following, it runs, but when you craft a stone chair, it produces two stone chairs a time (although sometimes you’ll get a fine version on one of the chairs after level 3).
{
"produces": [
{
"item":"stonehearth:furniture:stone_chair",
"fine":"yourmodhere:furniture:stone_chair:fine"
}
]
}
@Froggy’s Stonehearth Cafe does almost the exact same thing with a mixinto to a “” : [] style command. He adds new crop types with it. It is really strange that it fails here.