the error means that the Dreamweaver Site Definition is not set up properly.
In the Site Definition, double check the settings for connecting to the Testing Server, pay careful attention to the settings for the Host Directory and the URL Prefix, the error you are getting means that one or both of those settings is incorrect.
The URL PRefix should be the site address for your site. The Host Directory should point to the web root folder on your server.
If you copy the address used for the URL Prefix setting and paste it into the address bar of your browser, it should load your testing server site.
In the testing server Tab are you using an FTP Connection?
if you open the file panel (Window -> Files) and change the file view from local to testing what do you see? can you send a screen shot of the files panel list in a zip archive.