Any idea what I'm doing wrong? (Can't get anyone at Home Depot today
who knows how to operate one....grrr).

If you bought it from a John Deere dealer, the answer is just a phone call
away. Sounds like (engagement knob?) you are turning on the blades before
starting. JD have a disconnect that only allows you to do this when the
engine is running, probably prevents the engine from starting -- never tried
it this way on mine.