You can't input coupons on the European Avis websites. I called the customer service in Sweden and the girl I talked to didn't even know what it was, since they only use AWDs.

Try booking the same on avis.com, avis.ca, avis.com.au or avis.co.nz and see if you get a similar price, because there you can add coupons. Note however that coupons are usually limited to specific countries, so will be rejected if you it use for a booking in another country.