It looks like this is UA805, the additional 4x weekly SFO-ICN service that starts on 4/1. If so that should be a formality as the US and South Korea have an Open Skies agreement. I had the same message when booking the second daily SFO-SIN service (UA29) before it launched and the message disappeared the day of that flight’s inaugural.

The only reason I say “should be a formality” instead of “will be” is that the only thing I can see that would prevent it being approved would be another government shutdown. But that doesn’t look as though it’s happning so you should be fine.

Have a great trip.