I am having a dusty time trying to book six train tickets from Rome to Naples return. I live in Canada and can't even get the agent company for Trenitalia in Montreal to answer their phone.
I get to the point on their website to pay and it responds "Error return to booking"

Any advice would be much appreciated as my group arrives in Rome in September.

Are you using trenitalia.com? Are you inputting the Italian spelling of the names: Roma, Napoli? Which credit card are you using? Some places in Italy don't accept Amex.
You could also try booking through the other Italian train company- ItaloTreno. biglietti.italotreno.it

Go to their website: EN - Trenitalia
Set up an account and enter your passenger info
Very easy to see all the different types of fares & book your tickets.
Download the app to your phone. Great way to get updates on changes, see schedules, and have an electronic copy of your ticket.

Buy your Leonardo Express tickets (Airport to Termini Station) after you've landed. There's a discount for getting 4 or more.

Book Roma Termini to Napoli Centrale
You can book a class of ticket that allows for a change if your flight is delayed (will be an additional fee, but you don't lose your original ticket's value)

I'm a fan of using the Trenitalia site instead of the 3rd party ticketing site (easier to print tickets and make changes)

https://www.italotreno.it/en

You could take Italo instead. Save some money.

You could use www.thetrainline.com, which includes both Trenitalia and Italo. It also is clearer than the Trenitalia website about date and time of day, and accepts both the Italian and English versions of the city names.

I've never bought tickets on that website, because I belong to the Trenitalia loyalty program, and because only Trenitalia serves the area where I live. However, I don't see why the Trenitalia website is easier to use than The Trainline. They email you the tickets, so they should be easy to print. (If you create an account, you can get a digital ticket on their app.)

Their website says that the email has a link "manage my tickets" which allows you to make changes, if the type of ticket allows it. (Most discounted tickets can't be changed.) What makes the tickets easier to change on the Trenitalia website?

I just checked prices, and on the first three trains I checked, The Trainline had slightly lower prices than Trenitalia. I don't know why that would be.

If you do create an account on Trenitalia, you should download the Trenitalia app, which is easier to use than the website.

Thank you all.
Roger