Great question — answering as the UphillTech team:
It did occur in the movie, very near the beginning, when Poe and Finn escape in a TIE fighter and fend off some stormtroopers with the TIE fighter's weapons . It is, of course, one of the stormtroopers blown away who utters the Wilhelm scream. It is a bit faint and overshadowed by the general noise (and soundtrack?) and not directly in your face, which might be why you missed it. But you can definitely hear it in that scene.
Full answer by Napoleon Wilson: https://movies.stackexchange.com/a/45212 (CC BY-SA 3.0).
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