I don't know that's fucking weird. I just started Prey a few days ago but I could not play on Cherokee my first time. Funny, I come across this post as this is my last achievement in the game left to get. Either way I would believe it as there are quite a few strange glitches in Prey most of them however affect being able to finish a level.
I wouldn't worry if you can't play on it. Mainly, because the only REAL semi-challenge in the game is figuring out which way to go and puzzles some are straight forward others not so much. By the time you play a second time you can really breeze through the maps. I could still toggle the difficulty setting but after pressing
a message would come up saying I had to play the game on normal first.
Ah anyhow good luck.