I am trying to get the access token using the authorization code using this documentation Implement authorization by grant type | Okta Developer. This code I am getting from the web browser and I am coping the code and pasting into the postman.
There may be something wrong with the code challenge and verifier that you are generating. If you are making the API requests yourself, can you try using the example code challenge and verifier from the guide in a login flow to see if the issue is with your PKCE authentication?