Place it immediately after the failed moment, as the first reaction in the edit, then follow it with awkward crickets to hold the beat before a chipmunk laugh closes the scene.
About this sound
This is a very short voice clip, lasting about 1.3 seconds, of a man saying 'Aw man' and stretching the disappointment into a long, wavering 'aaaaa'. It feels instantly defeated and comic, like the sigh you make when a plan falls apart in front of you. The drawn-out groan makes the letdown sound bigger than the words alone.