The Eurail website is unreliable. Try to use other websites, you'll even save money (Eurail adds 2€ per person per train).

Follow this guide: https://www.seat61.com/interrail-and-eurail-reservations.htm

What route, date and time are you trying to make a reservation for?

On 5 & 6 September.  Am trying to plan leaving Rome Termini  as early as possible, and return by 10pm.

Firenze SMN to Rome Termini (1-way)

Rome Termini to Venice (2-way)

Thanks, read thru the guide and checked out raileurope.com.  But this looks to be for buying new tickets (single use c/w seat reservation).  Will explore the other options as well before I throw in the towel  😅

The linked pages are specifically for passholder reservations but you need to follow the instructions carefully.

On raileurope you must use their desktop website, if using a tablet or phone then your browser should have a setting to force use of desktop site.

Only on this version of raileurope is there an option to add your pass, once done it will then show passholder reservation prices for your search.

Thank you for pointing me to raileurope website.  Making seat reservation for two travellers with assigned seats was a breeze.  However, there is one route which I realised I booked 1st class seats instead of 2nd class (my pass).  Any idea how to go about changing them to 2nd class?  I’m just afraid we will be asked to leave the coach due to different class coach booked.  Thanks.

Get a refund and book new ones. It should be possible but depends on the route/company.

Yes, you could even be fined for travelling in the wrong class.