Answer:
Stacy got to go on a vacation to the beach, but it rained every day she was there.
Explanation:
You have to put a "comma" before the coordinating conjunction "but" because "it rained every day she was there" is an independent clause. This means that it can stand on its own and it expresses a thought, so you can understand it even without the first clause.
On the contrary, although the "dependent clause" has a subject and a verb, it cannot stand on its own like the independent clause. If the conjunction is followed by a dependent clause, there's no need to put a comma before it.
