When using DocuSign Connect, you don't need to do anything in the receiving application to explicitly acknowledge the Connect POST and respond to Connect. Connect simply relies on basic HTTP protocol semantics -- i.e., it sends a Request to the endpoint (URL) you specify in the Connect configuration settings, and if that web server successfully receives the message, then it will automatically send a response back to Connect (i.e., per basic HTTP request-response protocol).
This error in DocuSign Connect simply means that the message Connect is sending to your endpoint is not actually reaching your endpoint. I'd suggest that you check the following:
- Is the URL that you specify as the receiving endpoint a valid URL?
- Is the receiving endpoint behind a firewall? If so, have you done what's necessary to allow Connect messages to pass through the firewall?