The only way I've seen to be able to use an award certificate is to choose to pay with points and then it will let you. However, if you only have 1 certificate then it seems that you have to have points or buy them to pay for the other night.
If I want to use a certificate along with a paid night do I just have to do 2 1 night reservations? 1 paying with points that will take the certificate and then one paying cash? Then I suppose I would most likely have to switch rooms, but I guess that's better than switching hotels (which I'm also considering 2 nights each at 2 different hotels).
I'll start by saying, I don't know the answer to how to mix certificates with paid nights using the marriott.com grid (I do know that if I want to modify a certificate stay, by rule, I must contact a rep), so I will be interested in what others have to say; but since I'm on the forum responding to something else, I'll jot a few quick thoughts before I grab my snow shovel and head out.
Ever since the website inexplicably used one of my Cat. 1-4 certificates in a non-chronological fashion (used one of my newest ones first - these rascals are difficult enough to find use for - I don't need to shorten their lifespan), I have now decided to exclusively use the plat line (I see you're a gold, so whatever phone line applies, I suggest using). With a human, I have been able to mix and match certificates and paid nights, using the exact specific certificate I requested. This also allows me to use the certificate for the exact night I want and pay for the exact night I want (which of course applies when booking over weekends - pay the weekend, use cert. for the more expensive weeknight*). So in summary, I suggest using the human touch and then immediately checking your confirmation (it will show how your reservation is paid for by night). Good luck, off to the driveway.
and now a private note of personal paranoia to the world renowned author formerly known as steppingstones
*@anadyr - this is what I'm talking about, regarding how Marriott will (not even would, but will ) eventually price our rewards more aggressively if it was by room price - weekend rate 20,000 points for a Cat. 4, weeknight rate 22,750 points).
Since I knew I wanted to use the CAT 5 certificate when I go to the Renaissance IRS in Vienna in June I went ahead and booked it. Then I went back to what I initially trying to book & the system took the CAT 4 certificate since it was the only one left. I'm sure calling would be best, but I hate talking on the phone.