I had a similar situation when trying to book BF to HNL. The site showed 3 standard BF awards available, let me select the flights and identify the passengers. When I tried to select the seats, I got the error message.

I called co.com tech support. Luckily for me, the person on the phone took the initiative to "call headquarters" and got permission to release the seats to me for that mileage level since the website started that they were available.

Maybe try again and see if you get a different answer?