I've encountered a few hotels that do not let you book a Sat night as a one night stay. A few reasons for this, which i won't bother going in to.

IHG certs must be used one at a time, so any hotel blocking one night reservations will be automatically blocking the use of free night certs. They are not picking on free night certs.

AFAIK the only way to beat this is to call the hotel front desk and explain the situation, then hope they will make an exception for you.