There are a number of possible reasons. The most likely is that this is a non-refundable credit. It is used to reduce your tax liability to zero. But if your tax liability is already at or near zero then there is no credit to be taken.
The maximum credit for an air conditioner like that is $1200. It has to be taken in the year that it is purchased and can not be carried forward. If you owe no income taxes then the credit has nothing to reduce and does not get used.