Definition - What does Unknown Host mean?
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
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.