Marriott's system will allow you to "link" a valid cert to a reservation even if that cert will expire before the actual stay. The problem is that when the cert expires, it will disappear from the system and leave the reservation without a valid cert. Then about a week before the actual stay, the system will automatically try to deduct the required points from your account to order and link another cert to your reservation.
If you don't have enough points, Marriot may try to contact you to see how you want to deal with the reservation (pay for more points, convert the reservation to a paid stay, etc.) If they can't get ahold of you, you might be on the hook for what ever cash rate the hotel wants to charge you (which might well be more expensive that you would want to pay.
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Of course if the cancellation date has past and you can't get a valid cert attached, you will be on the hook for the cancellation fee.