How to save iptables configuration on Redhat Enterprise Server 7

You should install iptables-services package.

Then service iptables save will work. Also these commands will work too:

# iptables-save > /etc/sysconfig/iptables
# ip6tables-save > /etc/sysconfig/ip6tables

AFAIK, systemctl doesn't have any option to save iptables-services' configuration.

Note: systemctl syntax is as follow : systemctl <operation> <unit>