This topic has come up before, but I wanted to report how I successfully bought tickets to the Pantheon today to save someone else some time and frustration.
The site I used was the Italian Ministry i of Culture: https://museiitaliani.it/en
I tried to use my credit cards (multiple cards, multiple times). They did not work. The credit card company had no record of the failed transaction.
I finally succeeded when I used PayPal. For PayPal, click on “altri metodi di pagamento”
One other hint. I have not received tickets from other museums, etc. in Italy when I used my regular email which ends in .net (I did receive the tickets when I sent a follow up email). This happened with the Uffizi and the Colosseum. I had no problems when I used my email ending in .com