repair_packages: command not found
You're getting that message because the repair_packages command no longer exists in Sierra. Repairing permissions, which is what they're trying to get your system to do, is automatically handled during software updates now. I'm not sure what the underlying issue they're trying to fix is, but repairing permissions on system files probably is not the issue anyway, especially on a clean install.

Apple's not going to be able to help you because Sierra doesn't have that utility. You'll need to go back to WebRoot and ask them how to resolve your issue on Sierra.

(For a company that claims to be selling internet security software, you'd think they'd be a little more versed in the OS for that software, especially before asking a user to run a command in Terminal!)