Kant’s critique of existence, as it filtered through Analytic philosophy, became an argument like this:
Any real predicate (a predicate said of individuals) must be able to be both affirmed and denied of individuals.
Existence cannot be affirmed and denied of individuals.
Therefore existence is not a real predicate.
I see the force of the argument but it seems like it works better as a reductio ad absurdum against the major premise. “Exists” and “real” are more or less equivalent predicates, and it would be very odd to demand that “real” and “unreal” both be real. It would be like demanding that there were no real places on maps because “not a place on the map” does not show up on GPS. In other words, we can just say (with Barry Miller) that existence is a real predicate and non-existence isn’t; or that the whole problem is in assuming that non existence must be a real predicate, not in assuming that existence is one.