When I tried to restart netflow collector I saw message: "Error: unknown parameter restart"

This message is displayed when you run the command nfcd restart without /etc/init.d/ prefix. Please, run this command with full path. Correct command is:

/etc/init.d/nfcd restart

Or you can run short nfcd without any parameter, but /etc/init.d/... syntax is preferred. After restarting collector, check your system log file (cat /var/log/syslog).