Who told you it was an error? How did this come up? Did you identify yourself, so that someone could prove that you were told this? Does the code itself show that it's expired?

Before people jump on me, I'm not trying do anything dishonest, but my attitude, and I think the law supports this, is that if it's on the website and not obviously old (like past a stated expiration date), it's bookable. Marriott is responsible for what's posted, not you, and I don't know if you even have an obligation to check. You have no way of knowing whether what you were told on the phone is accurate and the website is wrong, or the person who told you is just misinformed and the website is correct.

The two exceptions I could see are that 1) the website says something so absurdly cheap that you should not believe it, like well over 50% cheaper than any other available rate (I just got a 30% AAA discount at the Maui Kaanapali resort during what ended up as a sold out week, so it would have to be cheaper than that), or 2) after you book, someone contacts you and says it was a mistake and they won't honor it, if they're allowed to do that.

I think I would at least book it, and then try to sort it out before you show up. If you really want to go, maybe book another rate as well to be safe, and cancel one when you figure it out.