We recently had a similar situation using the Marriott Visa Certificate. Our online account had the same statement of "you have not ordered your certificate yet", and how many points would be needed to redeem this night.
When we got to the hotel, there was no issue, the certificate is valid for one night at a category 4 or less as long as the reservation was made using Marriott Rewards. I just think that the software is not configured to indicate anything other than Marriott Rewards when not using revenue.
Dash