"Wake up, everybody!"
Why not? Well, it is a perfectly acceptable sentence, and it has the right meaning. However, the course creators have established that words and phrases should not be rearranged without a grammatically required reason. That serves a couple of purposes: it cuts down greatly on the amount of work the maintainers have to do in order to accommodate all the potential permutations of answers; and it helps to verify that the student isn't thinking about Klingon word order as just a lazy reversal of English.