The correct sentence is a third conditional: would have + past participle / if + past perfect.
Remember that an if-clause can come at the beginning or end of a sentence. If it comes first, it is often separated by a comma.
It is used to talk about past situations which did not happen.
In your example, Stacy did not know about the party.
As to the wrong sentence, I don't think your Manhattan book is correct.
Student or Learner