As long as you've got it noted as "Planned" or "In Progress" you'll be fine. You don't have to take all of your pre-reqs before you apply, just before you start school, and in the Adcom's eyes, you haven't taken that pre-req yet. Don't stress, just do your best 🙂
And btw, I think the 20 really shows that you learned the stuff, despite the bad grade, and that'll be a redeeming factor for you there.
Well, if you're using it as a pre-req, you'll need to get at least a C in it. Just to clear the point up, by "schools," I meant undergrads, not dental schools.