I am in a similar position this evening. I have tried to purchase using three different cards, all of which give slightly different error messages ('gmfctickets couldn't connect' or '3DS authentication not available'.
The whole website is a shambles to be honest - having to click through a million options to select a 'seat' when you want to stand in the shed, being asked to load a 'seat selector' which never seems to appear on my machine. Generally it feels like a ballache to buy a ticket. I have used the contact us feature to flag my issues but can't say I'm confident it will get fixed. From memory I tried to buy a season ticket last year on the website a couple of days before the first game and basically gave up because, you know, not everyone lives within 15 minutes of Smiths...