I would guess that the Kauai Marriott has used up the number of standard award rooms it has alloted. (Note that this doesn't mean they are sold out of those rooms for paid reservations, only that they no longer have any left they will allow standard award redemptions.) What some hotels will do is they will not only allow awards for base standard rooms, but will hand out some "upgraded" rooms for award reservations. These room normally will cost 5K or $50 more per night per level of upgrade. IOW, a single upgrade from a garden view room to an ocean view room might cost 5K or $50 extra per night. A two level upgrade maybe to a higher level ocean view room might cost 10K or $100 extra per night. A three level upgrade possibly to an ocean front room might cost 15K or $150 extra per night.

Since elites tend to get free upgrades more than others, most of them when they are making award reservations will go for the base level awards and then hope to get a free upgrade. Unfortunately for you, this means that those rooms will tend to get reserved much more quickly than the "upgraded" rooms. So in your case, all the base standard rooms are gone with only the upgraded ones available.

You have a few options available to you. You could go ahead and make the reservation, pay the the difference in points or cash, and enjoy your upgrade. Since you are a Plat, you might go ahead make the reservation, try to contact the GM and see if you can negotiate something. You might get lucky and the GM will cut the "extra" cost down to 5K or $50 per night. Another option is to try different nights to see when the hotel still has base standard award room availability.