The service command supports only basic LSB actions (start, stop, restart, try-restart, reload,force-reload, status)

# service iptables save
The service command supports only basic LSB actions (start, stop, restart, try-restart, reload, force-reload, status). For other actions, please try to use systemctl.

With RHEL 7 / CentOS 7, firewalld was introduced to manage iptables. IMHO, firewalld is more suited for workstations than for server environments.

It is possible to go back to a more classic iptables setup. First, stop and mask the firewalld service:
 

systemctl stop firewalld
systemctl mask firewalld

Then, install the iptables-services package:

yum install iptables-services

Enable the service at boot-time:

systemctl enable iptables

Managing the service

systemctl [stop|start|restart] iptables

Saving your firewall rules can be done as follows:

service iptables save

or

/usr/libexec/iptables/iptables.init save