Hell @Rosa207,
To see travel credit when making a reservation the the reservation has to be a "Qualifying" reservation. The terms describe this as shown below. If the value of the accommodation charge for the reservation is less than this then the credit option will not appear when you make the reservation. If your first one was for more than this then contact Airbnb support.
You can't apply travel credit to a reservation that is already made.
Qualifying reservations
A qualifying reservation usually has a total value of $75 USD or more, excluding guest fees and any taxes. These amounts may vary during promotional periods.
Use your travel credit
Your available travel credits automatically appear as a coupon on the checkout page of your reservation request. If the travel credits aren't automatically added, it means the reservation doesn’t qualify.
Travel credits can't apply to a trip that has already been paid for, but they will be available and applied to your next qualifying reservation. They expire one year from the date they're issued and can only be used once.