For example, you can't connect to services in the Classic environment that use IP address 127.0.0.1, or "localhost" as the server address or host name. (Experienced users call these "loopback addresses," but they can also be thought of as meaning "this computer.")
What do I do?
Just open Network preferences and look at your Mac's assigned IP address or its host name. Use that instead of 127.0.0.1 or "localhost".