The client identifier provided is invalid

I am following this document(https://developer.zendesk.com/documentation/ticketing/working-with-oauth/using-oauth-to-authenticate-zendesk-api-requests-in-a-web-app/) to set up zendesk authentication. But I am encountering this error while exchanging code with token.

{"error":"invalid_client","error_description":"The client identifier provided is invalid, the client failed to authenticate, the client did not include its credentials, provided multiple client credentials, or used unsupported credentials type."}

Please let me know if I am doing something wrong.

Thanks
Anthony