This article is for anyone unable to enable hosting due to Parsec believing the computer is running a version of Windows below 8.1 or 10.
If you get this message when running Windows 7, it's normal, because Windows 7 can't host.
On the computer you're trying to enable hosting on, visit
Settings -> Hosting -> Advanced Settings
And add the below line to your text file, then save, and restart Parsec from the icon in your Task Tray (by your computer's clock).