I had this experience last year. Upon making my online reservation, the computer notified me I had an e-cert and asked if I wanted to use it. I did it, not realizing it would expire before my trip. I only realized there was a problem just before my departure. I called & e-mailed Marriott Rewards and all they would do is take points from my account to cover the room.
Only when I contatced Marriott Concierge here in FT, did it get resolved. I was able to use my expired certificate for the 1 night. Even then, when I checked out there was massive confusion that held me up about 20 minutes while the meter on my taxi was running outside the lobby.