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