Tell them the system isn't accepting it and there is no way for you to override it (because usually there isn't). Sometimes I show them the screen with the big red box if they aren't believing me. If the option is available, I usually do "accept anyway" because it's $1 or less and not worth getting the guest worked up.
Reasons:
1. Product is different than the coupon
2. Coupon is expired
3. Target doesn't allow coupons from that vendor/manufacturer