@Marie10645 generally Airbnb makes it hard to use credit and you need to do it at the point of booking. I fear you have an additional issue now in that if you have two overlapping bookings you cannot cancel the first and get your Airbnb fees back. All you can do is call Airbnb and ask them to sort it out - You may find they can't within their rules.
Mike and Helen shared some useful information, and if you still need to contact the Support teams you can do so via this page as well: www.airbnb.com/help/contact-us/