I've been trying to book using points at the Rome Marriott Grand Flora with no luck. I finally contacted Marriott and got this response.

Thank you for contacting Marriott. We appreciate the opportunity to
provide you with information and value your loyalty as a Marriott
Rewards member. Please accept our apology for the inconvenience you
experienced when attempting to redeem your Marriott Rewards points. The
Rome Marriott Grand Hotel Flora is one of five Marriott hotels that do
not participate or do not fully participate in "No Blackout Dates" at
this time. The information can be found at the following link:

http://www.marriott.com/rewards/terms/usetrms.mi#hotel

The entire list of hotels is included below:

- Rome Marriott Grand Hotel Flora, Rome, Italy
- Waikoloa Beach Marriott Resort & Spa
- Wailea Beach Marriott Resort & Spa, Hawaii
- Camelback Inn, A JW Marriott Resort and Spa, Scottsdale, AZ
- JW Marriott Capri Tiberio Palace Resort & Spa, Capri, Italy