Hello
I tried to reserve my tickets on trenitalia's website but unfortunately it said the payment was denied. I called my credit card company they said they have authorized the transaction when I called Trenitalia they said they don't not accept international credit card
so you can only purchase with Italian credit card which I find it very inconvinient

Correct. Trenitalia is not likely to process your card if it's American although at times one or two people seem to get their cards to work. You can buy your tickets when you get to Italy.

will i be able to find the exact departure time or it will be sold out ?
I am arriving to Rome on the 17th and leaving for Venice on the 23rd.

The times are on Trenitalia's website. It used to be they would only show schedules for 30-60 days for the fast trains and if you were taking a slower train (or combo fast and slow) you could only see schedules for the next 7 days. Schedules really don't change much. Just enter a date within the next week to get the times.

If you buy your ticket to Venice when you first arrive Rome, you might be able to get one of the discounts available for advance purchase. I doubt the particular train you want will be sold out (you didn't say when you were traveling) especially if you buy several days in advance.

A month ago I tried several times to purchase tickets on Trenitalia to no avail. I had been able a couple of years ago but apparently they don't usually accept US ccs anymore. I bought our tickets when we got there.

thank you guys
I am leaving in September
hopefully it won't be as busy as summer