Command not found: systemctl on Amazon Linux 2018.03

sudo service nginx status

just use the service command

In earlier versions of RHEL use the service command as explained in the documentation here.

# service service_name start

Therefore, in your case:

# service iptables start

You can replace start with restart, stop, status.

List all services with:

# service --status-all

1. First check what Amazon linux version you have runnng on using the command cat /etc/os-release
   2. If it is Amazon Linux Version 2, then it does support systemd and comes installed by default.
   3. If it is Amazon linux version 1, then it doesn't support cause Amazon Linux is ultimately based on an old version of CentOS/RHEL.

if it is version 1 (amazon linux), you just need to switch to any other linux distribution that supports systemd. You can't yum install systemd like a package