DO allows only 1 promo code per account. https://www.digitalocean.com/help/pricing-and-billing/promos-and-credits/

So if you’ve already added one promo code, you cannot add another.

Hello, all

If you navigate to your billing page you should be able to see a link which says Apply Promo Code.

In order to activate your package you need to get your personal unique code and also to add a valid payment method to your DigitalOcean’s account (credit/debit card) or add some funds using PayPal.

Please note that you have taken advantage of a different promo code at some prior point each account is only allowed to use one promo code during it’s lifetime.

Hope that this helps!
Regards,
Alex