This topic should help:

If you still have issues then be specific about what/when/where.

Read the post suggested above :) 

Or let us know which legs you are talking about to get a precise answer (Route,Date and if already known time & Trainnumber ) 

Thankyou for responses. We have navigated how to book and pay for seat reservations. Upon trying to pay, it states one leg of our journey is no longer available, but doesn’t state which leg? How do we find out please?

Again, please specify route, date, departure time and train details if possible. Otherwise we’re unable to help.

We have booked many trains, starting in Paris on April 25, ending in Naples mid May. It doesn’t specify which train booking is no longer available, when we try to pay for the seat reservations. 

@Moody  For how many persons you are trying to book? 1st class? 2nd class?

4 people. 2nd class

The first three trains in France you can book here: https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish

For which you need a pass cover number:

It will be cheaper to reserve all (domestic) French trains in once via B-Europe, since you only need to pay a handling fee once, instead of 2 EUR for every ticket.

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Note that you can do Luzern - Como S Giovane reservation free, if you switch in Lugano to a regional train. Not taking that much extra time. Saving you money.

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The trains to/in Italy you can book via: ÖBB https://www.oebb.at/en/

Go to the planner window, add your route. Click on “book tickets”. A screen will appear, for which you’ll need to fill out extra details. Add Interrail/Eurail as a discount under passenger details for each passenger, and click book tickets. Price is 10 EUR per seat reservation. Do NOT select seat reservations only.

As advised by all means choose a reservation-free itinerary to Como S. Giovanni, you're paying 85 AUD for nothing, literally 1 km in Italy.

If you've got time, I'd really advise you to take the Old Gotthard Line over the 57-km Gotthard Tunnel. Very scenic (hourly connection). Could look like this :

- Luzern - Bellinzona 11:18 - 14:00

- Bellinzona - Chiasso 14:16 - 15:06

- Chiasso - Como S. Giovanni 15:13 - 15:18

Or go all the way to Locarno and spend 1-2h there. Trains are always hourly.

- Luzern - Locarno 09:18 - 12:25

- Luzern - Locarno 11:18 - 14:25

- Locarno - Como S. Giovanni 16:25 - 17:35 (or another one)

Btw the 12:18 connection from Lucerne is often crowded as it's a Germany - Italy direct train. Please take another one if you can.

Another note : tight connections in Switzerland are the norm so don't worry

Use SBB app for connections in Switzerland : it shows platforms, compositions and how busy the train should be