pservice is a late addition to the packages feeds. Likely it's not available in the 18.06 version. But itself is a simple init script.
- Copy raw content of https://github.com/openwrt/packages/blob/master/utils/pservice/files/pservice.init to
/etc/init.d/pservice - Make it executable:
chmod a+x /etc/init.d/pservice
Then config and start like previously said