This error message means that the machine cannot recognize the ethernet cable connected to it. To trouble shoot the issue, please proceed with the following:
Have the caller verify that the Ethernet cable is inserted properly into the LAN port of the terminal and into the available port on the router or switch.
Have the caller verify that the RJ45 jacks on either end of the Ethernet cable appear to be in good working order.
Have the caller verify that they are getting a light on the router or switch at the point of connection (if applicable).
You can also guide the caller on how to make sure that they were using the correct type of cable for a Lan Connection Set up by having them check the end of the cable as an Ethernet Cable must have 8 copper lines/strips that can be seen compared to Phone Line Cables that have 4 copper lines/Strips.
If no success, replace the cable altogether (Cat 5, Cat 5e and Cat 6 cables are cheap and readily available in various lengths at hardware stores).
Please note: there is an article on how to re-seat the S300 power adaptor connector here.