An unknown host is an error message that generates when a destination computer or host server name cannot be resolved. The message indicates that the user's provided host server name does not exist or match any Domain Name System (DNS) records.
The unknown host message is common when a user tries to connect to a remote host. The error occurs for a number of reasons, including underlying configuration, server unavailability or incorrect host name.
Techopedia explains Unknown Host
An unknown host message is generated when a user attempts to ping a host name without a configured DNS resolution. If the ping is unsuccessful, the user should verify that the ping was sent to the correct remote host address. If so, the user should verify DNS name resolution, configuration and Windows Internet Naming Service (WINS) availability.
Commands are often used to deduce the root cause of an issue. For example, the "nslookup" command tests host name resolution and verifies DNS server registration. This command usually follows testing a computer's Fully Qualified Domain Name (FQDN) host name resolution, where the computer's Internet Protocol (IP) address is located and verified.