You have given a hostname to VNC Viewer, but it's unable to resolve the hostname to a valid IP address. Check to make sure the hostname is correct and that it resolves properly. (You can do the latter simply by pinging the hostname; even if it doesn't get any responses, as long as it shows an IP address it's trying to reach, then DNS resolution is working.)
If the hostname does not resolve, you have a few options. You can just use the system's IP address instead of its hostname, assuming you know the IP address. If the hostname should resolve but isn't, there may be an issue with your system's DNS resolution or simply its network connection. Check to make sure both of those are working.