Hello @Joyce132,
If the option to enter a coupon doesn't appear on the page where you pay then it's probably not possible to use one.
You could contact Airbnb support for further information about this.
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.