Hello @Helen80,
My understanding is that one of the requirements for booking is that a guest has to have registered a payment method, e.g. credit card, with Airbnb.
The article below lists it as a requirement for booking.
https://www.airbnb.ie/help/article/1170/what-are-the-requirements-to-book-on-airbnb
For the ways to contact Airbnb support type "contact airbnb" in the "Search the community" box on these forum pages where you posted your question. Scroll down a little in the list of suggestions that appears and click on the "Contact Airbnb: A Community Help Guide[UPDATED]" one. That lists phone numbers and ways to contact them via Twitter or Facebook.
Steve.