Annoying, but not entirely unexpected.

There are a good number of threads and posts on here about VS being the poor relation when it comes to booking reward seats on partner airlines.

We have never been able to book Business or F rewards on SQ with FC points, and economy availability has been sparse / hit and miss at best.

You could keep trying, but there is a danger you will waste the Amex points, as people who have transferred over hoping book rewards have been unable to re-convert when the booking is unsuccessful.

Nick