UA Travel Discount Coupon for UA-Only Flights?

I received as a present a UA Customer Care Kit that my dad received. 1 of them is a 10% off travel discount coupon. When I searched for flights I could use it on, it seemed to only work on flights that were UA operated. If there was one LH leg, it would give me some "your search parameters are invalid" spiel or something.

Are these coupons for use on UA metal only? If it's UA "operated by LH", it wouldn't work?