Just a note to anyone going to Barcelona this August--tickets just went on sale for SF for the month of August. I thought I'd share my purchase experience, since there have been some posters who have struggled to have their purchase go through.
I used a Capital One Visa card. When I went to pay, the site required authentication so I had to either get a text with a code or hold my physical credit card to my phone. I've done it both ways in the past, and I chose to just get the text. Text came trough in seconds, I entered the code, and completed the purchase. No issues at all. Make sure you have 2FA set up ahead of time for your credit card.
Good luck!