This message occurs in Dynamics GP versions of 2010 and later, as the eConnect mechanism was moved to a WCF service.
This is usually indicative of eConnect not running on your server. Open "Services" and verify that eConnect is started and running. Verify that the startup is "Automatic" and that the service is currently running.
If your eConnect was working fine and then suddenly stopped with the above error (often following a reboot), the odds are high that you are using an expiring login account for your eConnect service. That means this problem will happen again as soon as the account expires. As stated in the eConnect installation guide, you should set this account to NOT expire, and you should consider minimizing the account privileges.