Do you need to pay to use the API?

Recently I implemented a simple application to send request with human text to the openAI api and then get back the result I want by callling openAI API, then i got following error :

Error communicating with AI: You exceeded your current quota, please check your plan and billing details. For more information on this error, read the docs: https://platform.openai.com/docs/guides/error-codes/api-errors.
Error processing the file: AI failed to interpret the request.

from the link of errror-codes, this is the cause of the error :

429 - You exceeded your current quota, please check your plan and billing details Cause: You have run out of credits or hit your maximum monthly spend.
Solution: Buy more credits or learn how to increase your limits.

Even i subscribed chatGPT Plus with 20 $ per month, i still can not use openAI api for granted.
I need to extra pay for the usage of openAI API