Not sure there is anything you can do but change your dates. Play with the dates. Sometimes its just a matter of moving up or back a day. I have had to pay for the first day then start the rewards stay. It will show unavailable if the whole stay can not be redeemed as a reward stay. I did have an experience recently that my first night was not available on rewards and the Marriott rep did call the hotel and get it approved. If you haven't already, give them a call. Also, ck daily, it could still show up.