Hello @Andrea936,
If you've already tried the suggestions below then you'll have to contact Airbnb customer support.
For the credits to apply you have to be new Airbnb customer and have set up your new account by following the instructions at the referral link. If you already had an Airbnb account, or set up your new account independently of the instructions at the link, then the credits aren't valid.
If you did set up a new account via the link then is the reservation you're trying to make a qualifying reservation? The reservation has to have a value of more than about US$ 75 before fees and taxes, otherwise credits will not apply to it.
You can find ways to contact Airbnb customer service by clicking on "Help", then on "Visit the help centre" and then on "Contact us".
Steve.