If you are trying to connect to your localhost then it will not work.
Connecting a Windows viewer app from the same primary machine is not supported.
You can only connect a Windows Viewer from another secondary machine to your primary machine.
If you want a local host connection, you can only do that using spacedesk HTML5 Viewer which operates thru web browsers.
Just open a web browser on your primary machine (e.g. Google Chrome) the go to http://viewer.spacedesk.net/, then try to connect to its own IP address.
Please keep in mind that spacedesk allows only 1 HTML5 viewer connection at a time.