The railplanner app is not complete with all the trains you may take. If a train is missing you can use it (if it is a carrier that partecipates). 

With a paper pass write this trip on. With the mobile pass you have to add manually the missing train. 

In addition, don't use the rail planner app to plan because it's not reliable: it doesn't get enough updates. Use the operators’ websites and apps. For international planning, the DB website or DB Navigator app is a good choice.

I agree with the answers above and I’d like to add: Even if you have an Interrail Pass usually you have to make a reservation for Italian trains and you’ll have to pay for it. 

From Tlse to Prs there are also IC trains via the non-TGV route=via Limoges. These also need an obligatory REServation, but its usually much easier to make and often just the 10 eur-a TGV runs via Bordeaux and is often much harder to find a seat on. In most seasons there is now also an overNIGHT train=IC de nuit, between the 2, coming from further south and also having reclining seats and couchette space

Note that as from this week eating/drinking is not permitted anymore on SNCF and also rules re pass sanitaire and wearing masks is much more stringent.

For domestic=inland trips use the local national planner of the railway concerned.

Whithin IT there is also a competing private HS-train operator, ITALO, which does not accept Eu/Interrail but may offer lower fares in the end.

In FR the strike-prone national burocratic operator SNCF now also does lower-cost OuiGO TGV trains, these do NOT accept IR/EU and have rules like the low-cost airlines. perhaps they are still undecided what trains they want to run for Tls-Prs. USA-ers tend to worry about these things in general too far ahead, often so early that timings are not yet available.