So, my friends and I are new to the game and have run into an issue. Some of the enemies we've encountered... specifically an oni, had armor with "Spiritual 5" as a quality on its armor. We can't figure out what this means.

Does this mean that it works for anything arguably spiritual?

Or is this a typo and it's supposed to be Supernatural 5 since this is the only other damage type aside from Physical?

Or is there another keyword or reference we're missing somewhere?

Good catch. I assume that this in reference to the Sinister Oni on page 320?

Yeah, that looks like it's a typo and in this case "Spiritual 5" should be "Supernatural 5". The oni would have both Physical and Supernatural resistance. The closest item quality I can find in the core rulebook (pages 240-241) to "Spritual" is "Sacred" and that wouldn't make sense in this context.