From one of your last questions , I assume that you are sending an API call via HTTPS from your server side.
You get this error because your server does not recognize the credentials for authenticating messages sent back to Google Checkout.
Google predicts all callbacks and notifications with basic HTTP authentication, using your Merchant ID as your username and your seller key as a password.
The HTTP 401 error indicates that you have not completely completed basic HTTP authentication on your server.
You may find this post helpful, which includes sample code on how to handle this situation: