Not all codes, discounts, coupons and/or special offers will work on all rental car rates
(there are usually a lot of terms and conditions that must be met in order for the discount to apply).
If the rate that you've booked is already discounted, the rental company may not offer an additional discount with the rental car promo code that you found.
It may also depend on whether or not the rental code has any restrictions or conditions that make it not compatible with the booking that you have reserved:
I would suggest that you call the toll free reservation line for whichever company that you have reserved with to see if they can add the rental car promo code or information to the reservation that you already have booked... that way you'll know in advance if it will work with your already reserved rate, and they'll be able to tell you if it can be applied to the reservation now, or whether you just have to bring proof to the rental counter for when you pick up the car.