I've been doing a bit of "research" in the Qatar manage booking section to try to request seating for our Sydney to Oslo flights. I think the reason I get the error message is that Qantas or Qatar has used the same Qantas FF/Club number for both my wife and I. I was able to request seats for me but not for my wife. I tried to update the details in the booking, to insert my wifes correct FF number but kept getting messages asking me to update all passengers details, which I had already done. It also defaulted to Christmas Island for our contact phone numbers, (maybe I'm about to be deported!!). Have to say it's a most user unfriendly web site even though I'm pretty computer literate. So I have to admit defeat and will phone Qatar. Good luck with flight search everyone.