I was having the same problem but after switching my credit card, it worked on a different card. I'm just guessing that my bank put a block on the first card. I had already made one international flight reservation on that card, but then canceled it within 24 hours. So this would've been the second reservation in 24 hours. I'm guessing that sent up an automatic red flag for them. Very annoying as I wasted about an hour in limbo trying to book the flight to no avail. But the second card went straight through.