KeyError: 'access_token' during OAuth 2.0

I'm building a simple application to learn Python 3 and Flask. The goal is to consume data from the Spotify API and for that I need to authenticate using OAuth 2.0.

I'm able to provide my credentials to Spotify Accounts however during the callback the following error is happening:

File "app.py", line 59, in callback
    access_token = response_data["access_token"]
KeyError: 'access_token'

Code sample:

post_request = requests.post(SPOTIFY_TOKEN_URL, data=code_payload, headers=headers)

response_data = json.loads(post_request.text)
access_token = response_data["access_token"]
token_type = response_data["token_type"]
expires_in = response_data["expires_in"]
refresh_token = response_data["refresh_token"]

This is the request response sample from the API documentation:

{
   "access_token": "NgCXRK...MzYjw",
   "token_type": "Bearer",
   "scope": "user-read-private user-read-email",
   "expires_in": 3600,
   "refresh_token": "NgAagA...Um_SHo"
}

I'm quite lost. Not sure if it's something related to the API response content or how the application is parsing it with json.loads(post_request.text).

EDIT: After getting the HTTP status code I'm able to have an idea of the problem:

requests.exceptions.HTTPError: 400 Client Error: Bad Request for url: https://accounts.spotify.com/api/token

However I still can't figure it out what is wrong with my request:

authentication_token = request.args['code']
code_payload = {
    "grant_type": "authorization_code",
    "code": str(authentication_token),
    "redirect_uri": REDIRECT_URI
}
encoded_oauth2_tokens = base64.b64encode('{}:{}'.format(CLIENT_ID, CLIENT_SECRET).encode())
headers = {"Authorization": "Basic {}".format(encoded_oauth2_tokens)}
post_request = requests.post(SPOTIFY_TOKEN_URL, data=code_payload, headers=headers)
post_request.raise_for_status()

Are you sure you actually getting data from Spotify Web API and not error response?

This is what I use and it works well

@dataclass
class Tokens:
    access: str
    refresh: str
    expires_at: int

# Class that sets up authorization_headers
# Class method
def get_tokens(self, code_from_redirect_uri: str) -> bool:
        payload: dict = {'redirect_uri': self._redirect_uri, 'code': code_from_redirect_uri,
                        'grant_type': 'authorization_code', 'scope': self._scope}
        response: Response = post(OAUTH_TOKEN_URL, data=payload, headers=self.authorization_headers)
        # When we don't get the OK response - we assume code was bad
        if response.status_code != 200:
            return False

        token_info: dict = response.json()
        self.tokens = Tokens(token_info['access_token'], token_info['refresh_token'], token_info['expires_in']*1000 + time())
        return True