@homecat
We cannot help you much, since this is the community forum, but open a ticket through the control panel. The support team has the possibility to see why that promo code doesn’t work with your account.

Hello, all

If you have no promo code currently applied to your account, you should be able to enter one within the billing section of your DigitalOcean control panel.

If however, you already have a referral code or other promo code active on your account, this option will be hidden. Should you wish to apply an additional promo code, please reach out to the support team who will be able to assist further with your account :)

https://www.digitalocean.com/company/contact/#support

Hope that this helps!
Regards,
Alex