Apple signin/login with django and flutter problem - invalid_client response

I'm having an issue with implementing Apple Sign-In/Login in my Django + Flutter application. Here's the flow I'm using:

1. On the Flutter side, I'm using the following code to get credentials from Apple:

SignInWithAppleButton(
  onPressed: () async {
    final credential = await SignInWithApple.getAppleIDCredential(
      scopes: [
        AppleIDAuthorizationScopes.email,
        AppleIDAuthorizationScopes.fullName,
      ],
    );

    print(credential);

    // Now send the credential (especially `credential.authorizationCode`) to your server to create a session
    // after they have been validated with Apple
  },
);

This returns information like identity_token and authorization_code.

2. For test purposes, I send the authorization_code in a POST request to the endpoint https://appleid.apple.com/auth/token with the following details: Headers:

{
  "Content-type": "application/x-www-form-urlencoded"
}

Body:

{
  "client_id": "com.example.myapp",
  "client_secret": "example_client_secret",
  "grant_type": "authorization_code",
  "code": "my_example_authorization_code"
}

3. For generating the client_secret, I use the following Python script:

# Load the private key
with open(PRIVATE_KEY_PATH, "rb") as key_file:
    private_key = serialization.load_pem_private_key(
        key_file.read(), password=None, backend=default_backend()
    )

header = {
    "alg": "ES256",
    "kid": KEY_ID,
}

payload = {
    "iss": TEAM_ID,
    "iat": int(time.time()),
    "exp": int(time.time()) + 15777000,
    "aud": "https://appleid.apple.com",
    "sub": APP_ID,
}

client_secret = jwt.encode(
    payload, private_key, algorithm="ES256", headers=header
)

print(client_secret)

After all that steps, every time I send the POST request to Apple's /auth/token endpoint, I get the following response: {"error": "invalid_client"} Is there something wrong with my flow? Could it be that I'm sending incorrect data, or should I be handling this differently? Any insights or suggestions would be greatly appreciated :)

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