I encountered the same issue with obtaining the authorization code, this requires a few key steps to resolve. Firstly when making a proper GET request to retrieve the auth_code, the response will contain a key called code. The value associated with this key is what you need to pass into the payload as the code parameter.

However, to successfully obtain this response, you must perform the authorization request correctly. This involves creating a front end where the user can approve the app’s request to access their personal information. Spotify's API will only return the expected response once the user manually consents to this access. Without this approval step, Spotify will not provide the necessary authorization data.

A reference to how this is done is this video: https://www.youtube.com/watch?v=olY_2MW4Eik&t=90s&ab_channel=ImdadCodes