If you cannot find your voucher, it may be because it has already expired, or because you have it in a different Fever account.
To ensure it is not the latter case, please make sure that you've logged into the app using the same login method you used to make the purchase (email, Facebook, Google or Apple).
You can check the expiration date of your voucher (and any other conditions it has) by tapping on it. Vouchers disappear once they expire, so make sure to check the expiration date ahead of time!