Stand-alone machines with no Ethernet port connection get this error when the system tries to access the network.
If this message is continuous, try to save any work to a local disk.
When a machine is configured as a networked system, it must be plugged into the Ethernet with either a twisted pair J45 connector or thicknet 10Base-T connector (depending on the building's Ethernet cable type).
Older workstations have a thicknet connection on the back, instead of a twisted pair Ethernet connection; therefore, they require a thicknet to the twisted pair transceiver to translate between cabling types.