Join Date: Jul 2007
Location: San Francisco/Sydney
Programs: UA 1K/MM, Hilton Diamond, Marriott Something, IHG Gold, Hertz PC, Avis PC
Posts: 8,147
What dates? Without that, it's just a guess as to why you can't buy this ticket on UA.
Looking at some random dates (eg, out Sept 18, Back Oct 2), UA will sell you these flights for $1333 (even less in Basic Economy), priced as a single SEA-SKG "T" fare so no issues with combinability/end-on-end /etc as these only apply when trying to mix fares together.