I hope this helps
https://www.marriott.com/help/rewards-faqs.mi
What if I do not have enough Points for a Free Night Award when I book my stay?
If you do not have enough Points for the entire stay, you can still reserve your room at select hotels. When you make your reservation, you must select "Use Points." Your room will be reserved, however, your Free Night Award will not be processed. Instead, you will receive an email confirmation of your room reservation only.
From the time of booking to at least 14 days before your arrival, you will either need to purchase Points (maximum 50,000 Points per account per year), or have additional stays to earn enough Points to cover the difference. If there are not enough Points within 14 days of the stay, the Free Night Award will be forfeited.
Once you have enough Points for your stay, you can order your Free Night Award online. First look up your reservation and then use the link to order your Free Night Award.
Your Points will be redeemed for your stay, and you will receive an email confirmation of your Free Night Award order along with a reminder of your reservation information. The hotel will have your Free Night Award on the reservation.