Amazon Cognito Token Endpoint Returns invalid_client

Hello,

I am using Amazon Cognito with Authorization Code Grant with PKCE. When I attempt to call the /oauth2/token endpoint, it returns {"error":"invalid_client"}. I have checked a couple of similar posts (here and here) but still have the issue.

Here is some sample code:

import base64
import requests

AMAZON_COGNITO_CLIENT_ID = "example_client_id"
AMAZON_COGNITO_CLIENT_SECRET = "example_client_secret"
AMAZON_COGNITO_TOKEN_ENDPOINT = "https://example_subdomain.auth.us-east-1.amazoncognito.com/oauth2/token"

def test_auth(auth_code, code_verifier):
    auth_data = base64.b64encode(f"{AMAZON_COGNITO_CLIENT_ID}:{AMAZON_COGNITO_CLIENT_SECRET}".encode())
    headers = {
        "Content-Type": "application/x-www-form-urlencoded",
        "Authorization": f"Basic {auth_data}",
    }
    body = {
        "grant_type": "authorization_code",
        "client_id": AMAZON_COGNITO_CLIENT_ID,
        "code": auth_code,
        "code_verifier": code_verifier,
        "redirect_uri": "http://localhost:5000/auth_redirect",
    }
    response = requests.post(AMAZON_COGNITO_TOKEN_ENDPOINT, headers=headers, data=body)
    print(response.text)

if __name__ == "__main__":
    auth_code = input("Auth code: ")
    code_verifier = input("Code verifier: ")
    test_auth(auth_code, code_verifier)

The output is:

{"error":"invalid_client"}

There is no further description of the error. The authorization data is generated as described here. The code verifier (shown) and code challenge (not shown) were generated as described here and here. The redirect URI is the same sent to the /oauth2/authorize endpoint.

Thanks in advance for your help.