Quo is not a valid Scrabble word for 2 distinct reasons:
- It is part of a multi word phrase, and never appears by itself
- It is foreign
Zen is also a loser on two fronts: foreign and capitalized. It’s common enough that it might have lost its foreign status, but the capitalization still keeps it out.
Injustice is good, but the Official Scrabble Player’s Dictionary only lists 2-8 letter words, so if you’re using that as your only reference source, you won’t find it.