Which date and time you are looking at for this connection ?

We have tried other dates and time but it stills says unavailable 

On OBB for Italian reservations you need to add interrail as discount and search for tickets NOT reservations.

You should by now even be able to tell the explanation all by yourself; too EARly. It is NOT rec to do that right 2 month advance-with such a flurry of trains-and even more with a change in Napoli, you do not need to be so overworried. It is really not very different from how it would work in GB itself. Do trains there sell out wholly so long advance? You are not competing for the few very cheap advance tix, just for 2 of some 500+ seats. AND IT is quite famous for its ritardo (guess now what that means?)

I went on the OBB website and searched for ticket with added discount. Plus, everyone is recommending  2 month. I even tried to see if it was just that date but no earlier days like next week and February is working.

As of today, this date is missing in the ÖBB timetable data for this train (it's possible until 05/03/23) so you can't do your reservation with them right now.

Other trains for this date are available. 

What's possible:

- wait until it becomes available

- choose an other train the same day

- choose an other way to reserve it

If you can't reserve this train for dates up to the fifth of march or if you can't reserve any train at all (?), you're just trying it the wrong way (see above).

 During 2022 I planned a few trains in Italy and this happened a few times - it will appear soon. As Mcadv points out there are many trains between Salerno and Rome so there is no reason to worry at this stage.

I find trains from Salerno to Rome on the 2nd of April.

I find trains from Salerno to Rome on the 2nd of April.

 

Yes, but not this one at 12.58 which is much faster than the other ones shown here. That's a problem of still missing data.

But I assume the OP has a general booking problem (?).

Trenitalia released some weeks ago the new timetable for after march. But not all trains in one time, so some trains got to ÖBB, some others are missing. This one is only some days ago got it on the timetable, becasue it is a ETR600 now running as Frecciargento, but they get now a redesign and get an upgrade to Frecciarossa. This is the reason because so late. 

Soon it would be possible to book via ÖBB 

No need to have a rush, there is a high speed train from Salerno to Rome every hour.