Yesterday, I ran into a cryptic error message in Xcode, and there were only two ten-year-old hits on Google.
Unsupported KVC aggregate in keypath
So, in case future you runs into this problem, here's another, different cause.
The short version was that I was trying to do an NSPredicate comparison for Core Data, meaning ==, and I'd accidentally typed = instead. I'd been working in SQL earlier in the day where = is used as a comparison operator, so an easy mistake to make in context.
let predicate = NSPredicate(format: "id = %i", buildingData.id)
let predicate = NSPredicate(format: "id == %i", buildingData.id)
It was initially confusing because I wasn't specifically trying to aggregate anything, but Xcode was clearly confused about what, exactly, I was trying to do.
Hope this helps!