Train from Monterosso to venice

It would be difficult for me to say without knowing what the error message says. The answer is probably there.

Can you share?

That route involves a regionale trip. They changed who can create an ID on their website. I'm not 100% sure but I believe "guests" can only purchase the high speed train tickets online, and that you need to be signed in if your run involves any regionale trains.

You should be able to buy the Venice to Florence portion.

ted,

You could try Italia Rail as their website is a bit more user friendly. One point to note is that there's no benefit in purchasing tickets for Regionale trains online, as there are no price breaks.

The easiest solution would simply be to research which rail trip you want to take and then buy the tickets locally at the Monterosso station. I've found that the agents there speak at least some English and it's a very easy process. At this late date, it's not likely you'll get any price breaks in buying online, so the cost will be the same as buying at the station (or a Kiosk). As some of the trains will be Regionale, DON'T forget to validate prior to boarding the train on the day of travel or you'll risk a hefty fine which will be collected on the spot!

You might be wise to purchase now on line if there are 2 of you. Trenitalia has a Saturday 2 for 1 special on its Freccia trains. So although at this date all of the Super Economy and Economy tickets are gone, you can get the 2 for 1 on many of the departures from Florence SMN. I see it is still available on the 11:30 and every subsequent departure on the half hour until 17:30, then 19:30, 20:30, 22:30. So if you have a traveling companion, pick a departure and buy. Then get there on the Regional trains with time to spare.
Edit, You do need to purchase 24 hours in advance.

Update. The 2 for 1 special is available on www.italiarail.com for 65 euro for 10:58, 12:54, 14:54, 16:54 departures from Monterosso.