There was a problem setting up iptables. You either have an older kernel or a broken iptables install, or ipt_owner could not be loaded.
Sound like you are running an obsolete OS and don't have any of the normal core basics setup which is actually common for older servers running CentOS 4 or earlier. Most newer servers have iptables enabled by default ...
Might try running this in your SSH shell as root: "yum -y install iptables coreutils binutils"
(If you are not running CentOS or Redhat, you may need to use "apt-get" in place of "yum" in the previous command)
Outside of the above, your server may be messed up a bit more than can be diagnosed here and might need someone to actually look at it to see what is going on with the machine.
If the above doesn't solve your issue, send me a private message and I'd be glad to give you a hand sorting that out and coordinate with you to take a look at what is going on with your machine.
As for "SMTP Tweak" itself, I would strongly recommend using CSF Firewall instead of "SMTP Tweak" in Cpanel; However, you can use neither of these if your base iptables is not working on your server since both CSF and Cpanel's SMTP Tweak are based upon iptables at least being installed and functioning correctly.
Anyway, like I said, I got your back on this one. If you need one on one help with this let me know.
Hopefully what I told you above helps you sort out your issue though.