@Ashleigh56 By definition, a voucher will never go to your bank account. If the credit has been issued, it should appear when you go into your Airbnb site menu > Account and click Payments. Scroll down below your payment method(s) and you should see a display of any travel credits on your profile, along with their expiry dates. Usually these are automatically applied when you place the next booking.
If your credit isn't displayed here, and also you haven't received an email containing a voucher code, you'll have to write to Airbnb for assistance.