I don't know if this is the same issue. But I had a similar issues a couple of weeks ago. I was trying to book an avois flight for my wife, my son, and I and it would not let me book for my son because he was he was not on my "Friend and Family List" (he is a newborn this would be his first trip), once I added him to my friends and family list I was able to book a seat for him. I don't know if this would solve your issue but it is worth a try. The Friends and Family list is separate from my household account.