At first blush it would appear that the front desk at the property is clueless. See the Marriott link below for details on the rules. Better yet, print out the rules and show them to the fdc or manager. Then, you could ask them to handle the problem.

Also, when asking for assistance, it might be helpful to name the property where you are having the problem. Please update here as to the outcome. Thanks.

"Instant Redemption Awards must be ordered at the hotel during the hotel stay to which they will be applied; they cannot be booked in advance of the stay."
http://www.marriott.com/marriott-rewards/use/instant.mi