Put it right after the impact in an edit, following a punch sound and before a fail sting, so the sequence reads hit, reaction, punchline. A bone crack works just before it for a harder fall, while a SpongeBob fail cue after it turns the moment into a joke.
About this sound
This is a short human scream, lasting under two seconds, with a single high-pitched 'ahhh' that rises and cuts off quickly. It sounds raw and urgent, like someone reacting to a sudden hurt, and it lands as startling and a little comic rather than frightening. That mix makes it work as an instant reaction to a fail, a hit or an embarrassing slip.