You have the screenshot filepath set to /media/ramdisk
I think that /tmp is a better location, as its a standard ramdisk.
Alternatively, the creation of /media/ramdisk should be included in the installation guide.
Cheers!
config.lua:

-- filepath where current game's last screenshot will be saved
-- ideally on a ramdisk (for speed and less stress on the hard drive)
SCREENSHOT_FILEPATH = "/media/ramdisk/mario-ai-screenshots/current-screen.png"