I thought it might have something to do with not putting in the 3 digit number on the back of the credit card.. but there is no where in the checkout section where they ask for the 3 digit verification code!
I've tried multiple times and re-checked my CC number and still it doesn't work! When I called up they said I would have to wait 24 hours to order again because I'd attempted too many times and it's a security issue.. But it's still so annoying seeing as I can't go into the store and get 20% !!!!
I had this issue with Joe's Jeans and I ended up getting charged 3 or 4x (however many times I submitted and got that error message). It was on my debit card too so that really hurt! Thankfully the whole thing was resolved and they credited my account right away.
I never figured out the problem since their CS never responded to me. I ended up just using a different card altogether.