PMS running on Nvidia Shield - can see library but can't play content on any device

That PMS version is from March 2017, if google is right… The current latest version is 1.14.1.5488 according to Plex Media Server - #243 by StSimm1 so something is keeping your system from updating PMS. I think you need to get to the latest PMS version… though I don’t necessarily think it’ll fix this specific problem, you’ll have a better chance of getting support.