Today I've been trying to buy the online reservation and "skip the line" from the Vatican website. All goes well until checkout, but at several different times, the system will not take my credit card payment. I talked to the credit card company (Chase), and my notice of travel in western Europe is in effect, and there is no problem of any type with my account. This is the first day I've seen tickets for my date available, but the site is allowing me to select preferred date/time.
Are there any caveats for using their site, such as phone number format or something else it is finicky about?