Please note:

1 Only for new DigitalOcean account that never used any other credit before

2 Need PayPal or credit card to verify the DigitalOcean account first to use this coupon

3 For DigitalOcean account stability, it is only related to the credit card or PayPal, not the coupon code. If the DigitalOcean account is not solid, do not use coupon code. For a solid DigitalOcean account, you can always ask DigitalOcean support to reopen your account even you have used the coupon code. The coupon code is legit by all means.

How do I enter a promo code? I went to billing but I did not see anywhere to enter one!
Please check

The code is to be redeemed in the DigitalOcean control panel on the Billing page.

NOTE: DigitalOcean only allows one promo code per account. This one-promo limit includes the referral program: In order to redeem the $50 GitHub code, you must not have used a referral code or different promo code in the past.

And DigitalOcean hides the promo code link sometimes. Need to contact DigitalOcean support if you cannot find it.