Thanks for advice. Don't really "need" the trip to SIN at the end, but have family there to visit and if the mileage is available no harm trying for a biz flight return.I think your itinerary is fine. Not all valid itineraries are bookable online. The system just doesn't like some itineraries for some reason.
Do you really need that trip to SIN? It's usually cheap and easy to get to SIN. Will you really be worse off by buying a return cash ticket to SIN compared to buying a one way ticket to BKK plus the hassles of booking a oneworld award over the phone?
Might try and book the Europe trip in the next month or so when reward available becomes available and then try my luck with the call centre later this year for the 2024 flights.