That's unnatural but better than "He can sleep for very long," which is nonsense. "He can sleep for a long time" is best.
Anyway, it's annoying that you can get through the lesson only by typing the answer in incorrect English.
This a strange way to say this in English - not normal usage. You might instead say, "She can sleep for a long time" or "She can have a long sleep."
"She CAN'T sleep for very long" would work as a translation of "Dia tidak dapat tidur lama sekali."