insufficient_quota isn't a rate limit, it means there's no billing on the API account. ChatGPT Plus is a separate subscription and doesn't include API usage.
Go to platform.openai.com > Settings > Billing, add prepaid credits ($5 is plenty to start). Keys may take a few minutes to pick it up; if it still fails, create a fresh key after adding credits.
Handle 429 differently in code: insufficient_quota = billing problem, don't retry. A real rate limit (rate_limit_exceeded) = back off and retry.