If you can't sleep because you are physically unable to sleep because of some reason it would be: 난 오늘 밤 잠을 잘 수가 없어. You could also say 난 오늘 밤 잠(을) 못 잘거야 like Sewon said which is a future tense that means I won't be able to sleep tonight/I cannot sleep tonight. If you're trying to say I can't fall asleep/sleep won't come to me it would be: (오늘 밤) 잠이 안와

"나 오늘 밤은 잠 못 잘거야" would do.