Each time I try to get tickets for the Last Supper, I get a message saying "service temporarily unavailable". Sorry for repeating this request but the Webmaster thought my question was answered elsewhere. If it was, I haven't found it.
Looks like you posted this under a different thread too?
https://community.ricksteves.com/travel-forum/italy/getting-tickets-for-the-last-supper
The website for purchasing Last Supper tickets seems to be quite fickle. However I was able to successfully reach the page allowing me to input my credit card information.