1. Where are you trying to make a reservation?
2. For which exact train(s) is this? (date and departure time)

You did not read the country-info; It clearly states that for INside IT you have to go to a real working person behind a counter to make these REServ. On the site you indeed get the normal full fare-or sometimes cheaper advance-fare info.

If you fancy it: you can also do this trip much slower and more changes (Napoli-Roma) with regional trains and see a lot more as out of the frecce=superfast hi-speed trains that mostly ride behind soundwalls-and for nothing extra with the pass itself

It’s the 12:50 train direct from Salerno to Firenze S.M.N on the 16th of March.

Maybe I did not read that bit, sorry. I would be happy to book on-site, but I’m not in Salerno yet. I thought it was better to reserve in advance? Will I be able to get a reservation there? And will it be any cheaper because I have the Eurail pass?

Thanks again

A reservation should be 10 EUR at a ticket office in Italy (it doesn’t have to be in Salerno). It can be done in advance on the Eurail website for 12 EUR per seat. There's no quota on domestic trains in Italy so reservations can be booked until the train is full. I had a brief look at the occupation and more than half of the seats are still available. 16 March is on a Wednesday, which is usually not too busy, so you'll be fine booking a reservation a day or so in advance.

Thanks everyone. I think my issue was that I did not realise my Eurail discount had already been applied! On Eurail it was 12 euro, but from the rail company it was much more! Hopefully this is helpful for other travellers, learn from my mistakes!

Yeah because therlcompany stopped to sell reservations for Eurail/Inerrail online :) 
If you had visited a staffed ticket office they would sell the reservation aswell for 10€ (without eurails service fee of 2€)