"Cannot load module" sounds like you did not correctly extract the archive. I suppose "hashcat -m <mode> -b" doesn't work either?
edit: support for ssh keyfiles was added after the 6.1.1 release. You have to use the beta version: https://hashcat.net/beta/
edit: support for ssh keyfiles was added after the 6.1.1 release. You have to use the beta version: https://hashcat.net/beta/