Hi!
"Je ne sais quoi" and "Je ne sais" are quite literary, they shouldn't be used in a talk. You can read these expressions in old books.
ex: "Je ne sais quoi faire. Je ne sais que lui dire."
"Je ne sais s'il va revenir"
Nowadays you'll mostly find "Je ne sais quoi" used as a noun, so be careful.
ex: Il a un je-ne-sais-quoi. (El tiene algo?)
So yeah, use "Je ne sais pas quoi" and "Je ne sais pas"!