Standalone 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 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, so they require a thicknet to twisted pair transceiver to translate between cabling types.