Maybe there is another FF number somehow associated already? If you can retrieve the booking and get the 'Add Venture Club/On Business' option, it sounds like the booking itself is OK and BA's system is picking up the lack of that number at least. I'd suggest you check it in www.checkmytrip.com and see what FF number(s) it lists? If it's a group booking or there's something odd going on, you may just have to give your EC number a quick call and get them to do it for you.

Phil