Did you start your local web server? Are you able to copy the web URL from the testing server defined in Dreamweaver into a browser and browse your site? The unknown error usually means no testing server defined or a bad web URL.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.