The ps command can be used to determine if your system is using init or systemd. If PID 1 is init, then you will use the service command. If PID 1 is systemd, then you will use the systemctl command.
If your system is using systemd, use the systemctl command to restart and enable slapd.
systemctl enable slapd systemctl start slapd systemctl status slapd
chkconfig slapd on service slapd start service slapd status