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

Which train requires a reservation?

For Eurostar/Thalys https://www.b-europe.com/EN/Booking/Pass#TravelWish

For a domestic TGV https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish
 

If you are Canadian you should have a Eurail pass and not an Interrail pass, unless you are living in Europe.

I am making reservation (ECE 9572) from Karlsruhe to Paris Nord on March 31, at 15:35

For the cross-border France-Germany trains see advice here :

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105/index3.html?postid=37040#post37040

If they are full, you can take regional trains to Strasbourg and then a TGV for 10-20€ (2€ booking fee on eurail.com) if there's availabilty.

Or regional trains all the way will edit]

Can I get TGV and regional trains to Strasbourg by my Eurail pass?

Yes you can.

Ok so I need just to pay for the reservation

Is there any phone number I can call and reserve seat? The websites do not have anything available!

That train is full, it's not necessary to call, they won't be able to help you.

There are seats on other TGVs from Strasbourg so the routing would be (earlier or later too) :

- Karlsruhe Hbf - Appenweier 15:39 - 16:20

- Appenweier - Strasbourg 16:41 - 17:06

- Strasbourg - Paris-Est 17:19 - 19:05 20€ seat reservation (+2€ booking fee on eurail.com)

It is an hour longer

Edit : or take a direct ICE from Karlsruhe at 11:32 or 19:32 (seats available)

@thibcabe linked you a tutorial where you can get the reservations for this route. All possible ways to get it, also by call. 

Check it out.

As I stated those site are for specific pass journeys, the first for Eurostar and Thalys trains only, the second for TGVs within France.

Sorry if I didn’t make it clear.

The links were very helpful. I could book a seat there but the interrail website was showing no availability!

This is why we direct you to the alternative sources wherever possible.

The Interrail tool tries to collate as many services as possible but often does not make it clear that their reason for not being able to supply is only a possibility and to try elsewhere.

Quite often in the planner they indicate that reservations are needed and indicate a cost, inferring they can supply them. In reality many have either not yet been released or even not available through their reservation service. 

The issue is often aggravated that it often goes through the process and fails at the end, even though the actual seat was not available. This was the point where you were advised the seats may have sold out. 

Sites like the B-europe ones tell you right at the beginning which trains have availability through them.

Eurostar (and possibly Thalys) have a restricted number of seats for pass holders on each train so you will often see availability for paying passengers but no availability for pass holders. Almost all other operators allow pass holders to reserve until all seats full.