I've read a few topics about this issue. We are heading to Barcelona next week for our first time. We have tried many times to purchase tickets on-line for Sagrada Familia, but have had no luck. After speaking with our bank each time they clear us for fraud alert, the issue appeared to be on Sagrada side of payment. I emailed their customer service and was told "the error that appears in the system is due to an incompatibility between the security system of our payment platform and the bank that issued your card". As we only have credit cards from this one bank, it looks like we'll be unable to visit Sagrada. (bummer!) We were able to get Park Guell tickets with no problem. In all the years we've been traveling to Europe, this is the first time we've run into this issue. For the hotel and other tours we have booked so far, we're good. I'm just worried we may run into this issue for something else while we're there. For those of you who have experienced the same problem....did you have any other sights/restaurants/bars, etc that had the same issue? Thanks for any insight you can provide!
Spain is occasionally problematic for me. Are you using VISA? I always make sure to have AMEX, VISA and MasterCard. MasterCard has occasionally worked for me when VISA has not. Also, oddly, AMEX has occasionally worked as well when VISA hasn't.
I'm sorry you are having problems, this would be very frustrating. IMO, a person traveling to Europe should have more than one credit card, preferably of different types. This doesn't help you now, but for the future.
I would contact your accommodation, and perhaps they could help you acquire tickets.