What should I do about my deceased brother's debt, which consists of installments for air conditioners, given that more than five years have passed since the sale and approximately one year since his death, and I have not found the creditor?

If you can reach the owner of the money or their heirs through official channels or newspapers to deliver their right, then do so. But if you despair of reaching them, then give the money in charity on their behalf. When the owner appears, they will have the choice between approving the charity or taking their money.