Coinbase has two documented mechanisms, and neither is a list of public codes.
First, the referral program: "The referral program lets you and your friends ("invitee") earn benefits when your friends sign up and make a crypto purchase using your personal signup link." The invitee must create a new account directly from the referral link and complete a qualifying purchase of any crypto via bank transfer, debit or credit card within 90 days of becoming trade-eligible. The bonus amount varies by country and is shown in the app. Important exclusions: purchases on Coinbase Advanced or Coinbase Prime do not qualify.
Second, promo codes exist as a mechanism - Coinbase's help pages describe "short alphanumeric strings (e.g., PROMOCODE1) provided by Coinbase that can be applied toward transaction fees for buy or sell transactions" - but they are issued only by Coinbase via email, push or in-app surfaces, are single-use, limited-time and not valid on Advanced/Pro. No code is published publicly, so this page does not pretend to list one.
Re-check
Both help articles are re-checked daily.