Put it at the exact frame a plan fails in your edit, then follow it with the SpongeBob sad song to carry the scene onward, keeping it first so the reaction leads the sequence before you cut to the next shot.
About this sound
Wallace Whimpering is a short clip built around a whimper from Wallace. The feeling is sad and a little pathetic, turning a small setback into a funny, awkward show of defeat. It works as a gentle plea for sympathy rather than a big, dramatic outburst, and that light sense of defeat is what makes it useful as a reaction.