Funny that is what you get in Linux Mint, given that it is based on Debian and on Debian (stretch) you get... $ openssl --help Invalid command '--help'

Funny that is what you get in Linux Mint, given that it is based on Debian and on Debian (stretch) you get... $ openssl --help Invalid command '--help'; type "help" for a list. Maybe Linux Mint 18 is based on an older version.