Hi,
This same error just started to occur on my Windows Plex Media Server, uncertain whether because of a recent Plex update, new XBox One or some changes I did on the computer.
However, I like to control my computer’s sleep patterns, and not the other way around! 
This solved it for me in a cmd prompt with admin rights, after every Plex menu setting proved useless:
powercfg /requestsoverride PROCESS “Plex Media Server.exe” AWAYMODE SYSTEM
Actually, it worked only with AWAYMODE, as well, but I like to control computer sleep with the excellent WinSleep app and only fire up the 300W behemoth when I need heavier services, like gamestreaming, HD transcoding etc., and leave the other lighter and trivial server tasks to a 1W RPi instead.
To wake the Plex Media Server when needed, one could use for instance the Plex WOL-plugin on an always-on embedded device, which uses a lot less power, or with other WOL tools and solutions.
This might not work for everyone, or be a solution to everyone’s liking. Just a friendly tip, without a personal need for a further discussion about the definition of a server, whether every server should always be on etc.
Have a nice day!
Br,
Vegard
