@Georgina-Yazmin0 I didn't think a guest could "schedule" the second payment. As far as I'm aware, Airbnb tells guests when the second payment will be charged, there's nothing for the guest to do except make sure there is enough money in their account to cover the second payment. And you can't decide to then pay it earlier than that. I think that's why you may have been getting that error message- trying to do something which isn't how it works.
And before you go cancelling the reservation, you'd best make sure to thoroughly read the cancellation policy for the listing you booked. If it's a strict cancellation policy, you'd only get 50% of the entire nightly fee back, and since you've only paid 50%, you would not be due any refund at all.
While I'm well aware that dealing with Airbnb can be extremely frustrating and angering, raging at a customer service person is the best way NOT to get help.