As Maestro Ramen said, Flying Blue does not like people creating accounts solely for the purpose of redeeming points. Creating a new FB account, which has never had any points activity posted to it before, more often than not gets flagged for fraud. In your case, the points were rejected before getting into the account. Consider yourself lucky. In many cases the ticket gets flagged for potential fraud, becomes unusable and then your points will be stuck in FB too. The only way for you to make this redemption is to first fly on a FB partner airline and credit points to FB. Once you build history with FB, then you should be ok to transfer from Amex and redeem points for tickets.

The suggestion for Delta is because they are a FB partner. So you could fly on Delta, and instead of using the Delta Rewards number, use the FB number to earn points on the flights. Once these points credit into your FB accounts, then you can try to transfer the Amex points.