I tried buying tickets, the tickets went into my basket and allowed me to enter the payment details, but when you click finalize payment the payment failed and asked me to try again. Tried different credit cards that I know work as well

I had a google and this seems to happen quite a lot to people on the Barcelona ticketing system as there were lots of posts with the same issue that I experienced. I know have several payments showing as pending, but no tickets, or any confirmation that the payment went through.