Jun 07 11:17:27 mail.aria systemd[1]: Starting ConfigServer Firewall & Security - lfd...
Jun 07 11:17:27 mail.aria lfd[6704]: Can't use an undefined value as a symbol reference at /usr/sbin/lfd line 7186.
Jun 07 11:17:27 mail.aria systemd[1]: lfd.service: Control process exited, code=exited status=25
Jun 07 11:17:27 mail.aria systemd[1]: Failed to start ConfigServer Firewall & Security - lfd.
Jun 07 11:17:27 mail.aria systemd[1]: lfd.service: Unit entered failed state.
Jun 07 11:17:27 mail.aria systemd[1]: lfd.service: Failed with result 'exit-code'.
System is the latest Debian 9 with all updates installed
I got the same message in Ubuntu server 18.04. I was able to fix it by changing TESTING = "1" to "0", but this make me not test the CSF before going LIVE so that can be a risky move. Something seems to be wrong if you can't test the firewall.
I agree that there is an error in the source code as lfd should fail reporting "Error* lfd will not run with TESTING enabled in /etc/csf/csf.conf" rather than attempting to start and failing when it tries to close and unlink the /var/run/lfd.pid file.
Bug aside, LFD will not run when TESTING mode is enabled.