The API is not based on a monthly fee, so you might be talking about ChatGPT the web chatbot that is $20 per month, or even some other service with a fake app.
API is not ChatGPT. API is billed for the amount of language data you use.
You must now purchase a prepaid credit, all non-partner accounts that were being billed at the end of month have been switched to pay-first billing.
If you click here and it says your “credit balance” is $0.00 (or less) then you must fund your account with credits first.