When I recently booked a place, I was given the option during the booking phase to pay in full which I took. Perhaps you missed that prompt and the default is to pay 50%. I wonder if you contacted Air BNB customer service if the system can allow the full payment. Here is a guide for contacting Air BNB.
Thanks Linda - Yes it transpires I missed the crucial prompt. But worth noting that your advice to contact them on chat was by far the easiest way to get an answer, rather than scrolling through reams of message forums! Cheers.