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

If your system is using init, use the chkconfig and service commands to restart and enable slapd.

chkconfig slapd on
service slapd start
service slapd status