Usually this error is caused by broken /etc/hosts file. The problem could be a missing localhost entry, syntax errors or incorrect whitespace.

For reference, here is what this file should look like by default on macOS:

##
# Host Database
#
# localhost is used to configure the loopback interface
# when the system is booting.  Do not change this entry.
##
127.0.0.1   localhost
255.255.255.255 broadcasthost
::1             localhost