I agree. Something's amiss. Call back and try again.

From the Anytime Rewards page:

"...you can spend just 50% more points for “Stay Anytime Rewards” that allow you to override blackout dates and other hotel restrictions to get the last room available. As long as the hotel still has a room available, it’s yours!

To make a reservation using Stay Anytime Rewards, please call 1-800-MARRIOTT."