Funny enough I am having the exact same issue with Sentinel, have my VPS on 16.04 because it's not exactly new, right? cannot get sentinel running at all.
This is my first attempt at a masternode and I'm not a Linux newbie - the instructions remain convoluted at best. It's amazing so many have gotten this far.
Now I have read that even with the watchdog errors it is paying out so I'm going to wait and see but this seems like a very basic issue to resolve.