Hi zzhou and welcome to FT!!!

As Kiwi Flyer said, you are trying to book a codeshare flight operated by a non-OW airline and this is possible only on the Global Explorer ticket (with restrictions), unfortunately not on the OW Explorer (xONEx).

In a xONEx, you can only include flights that are marketed AND operated by OW carriers. This means that you must book a flight under the code of a OW carrier and that this same service must be flown by an aircraft belonging to a OW carrier (operating carrier and marketing carrier may be different only if both are OW members).

The SEA-LAX flight you are looking for is marketed by AA, but operated by Alaska Airlines (AS), which is not a OW member. Hence it is not eligible for xONEx tickets. Same about the other Qantas (OW)/Jetstar (non-OW) flight.

The only codeshares possible under xONEx are those between OW carriers. You may book, for example, an Iberia flight from Miami to Madrid, even if the plane operating the service belongs to AA. Or a AA flight from NY to Hong Kong, a service actually operated by Cathay Pacific.