Spotify API responds with 'Invalid username' when trying to get currently played song

I'm trying to obtain the currently played song using the Spotify API. I read the documentation and it looks like you first have to authorize. As you can see, I'm opening the authorize page in the browser, and once I successfully authorize, I'm sending a POST request to the endpoint in order to obtain the access token which I later pass to another endpoint using a GET request. However, I'm getting Invalid username in return. What have I done wrong?

import requests
import webbrowser
import urllib

SPOTIFY_CLIENT_ID = 'spam'
SPOTIFY_CLIENT_SECRET = 'eggs'

def get_currently_played_song():
    webbrowser.open(
        'https://accounts.spotify.com/authorize?' + 
        urllib.parse.urlencode(
            {
                'client_id': SPOTIFY_CLIENT_ID,
                'response_type': 'code',
                'scope': 'user-read-currently-playing',
                'redirect_uri': 'https://www.spotify.com/us/'
            }
        )
    )

    response = requests.post(
        'https://accounts.spotify.com/api/token',
        data={
            'grant_type': 'client_credentials',
        },
        auth=(SPOTIFY_CLIENT_ID, SPOTIFY_CLIENT_SECRET)
    )
    access_token = response.json()['access_token']
    current_song = requests.get(
        'https://api.spotify.com/v1/me/player/currently-playing',
        headers={
            'Authorization': f'Bearer {access_token}',
        }
    )
    print(current_song.json())

if __name__ == '__main__':
    get_currently_played_song()

...prints:

{'error': {'status': 404, 'message': 'Invalid username'}}