Basically you need to generate new API keys. The ones that are tired to expired credit no longer work (and there’s no affordance on the UI for this).
TL;DR: You need to upgrade to a paid plan. Set up a paid account, add a credit or debit card, and generate a new API key if your old one was generated before the upgrade. It might take 10 minutes or so after you upgrade to a paid plan before the paid account becomes active and the error disappears.
As seen here