Most promo and voucher codes are created and issued by the activity provider or websites such as Groupon.
If your promo/voucher code is not working you should contact the issuer of the promo code (for example, Groupon or the camp or class provider) and let them know it's not working. The promo/voucher may have an expiration date or a minimum purchase amount and this may be the reason why it does not work for your purchase.
You can only use one promo/voucher code per activity provider per checkout. If you have multiple voucher codes to redeem for the same activity provider or business, you check out one at a time.
*If you don't know how to add your promo code, see this article.