OpenAI is using the same error code 429
for both rate-limiting issues and billing issues. Please refer to this article from OpenAI.
OpenAI Account Requirement
Make sure you have the right subscription plan with OpenAI. If you're having an issue with GPT-4, please note that this model is not available on the basic ChatGPT Plus plan.
⚠️ You need an account on platform.openai.com, not chat.openai.com
Other Errors
In some cases, you may get the following as well https://help.openai.com/en/articles/6891827-error-code-404-you-must-be-a-member-of-an-organization-to-use-the-api