There is a limited number of seat reservations for Interrail/Eurail passes on Thalys trains (quota). There might be regular tickets available while there's no Eurail seat reservations.
Let us know the travel date and we can help you for alternatives (there are many). There are usually more seats to Brussels (27€) or Lille by TGV (10-20€), the rest of the route on reservation-free IC trains.
Btw Interrail/Eurail's planner isn't reliable as it isn't updated that often. Check bahn.de or the train companies websites for planning
You can't book Eurail seats on thalys.com. Only regular tickets.
If you buy an (expensive) regular ticket in the end, then there's no need to use a pass day.
Seat reservations and the app are completely separate. Reservations won't appear in the app and you need to show both when required
You can check Thalys availability here : https://www.b-europe.com/EN/Booking/Pass#TravelWish
Thank you very much, I'm travelling 14 may
There are indeed no passholder seat reservations available anymore on 14th May between Paris and Brussels or Amsterdam. So you'll have to go via Lille It's a longer journey but it is straight-forward, don't worry. (This is an example)
- Paris-Nord - Lille-Flandres 08:46 - 09:48 10€
- Lille-Flandres - Kortrijk 10:09 - 10:45
- Kortrijk - Antwerpen-Centraal 10:58 - 12:23
- Antwerpen-Centraal - Amsterdam Centraal 12:44 - 14:35
You could suddenly decide if you'd like to stop in Gent or Antwerp as they're along the way. From Lille trains are reservation-free and run (at least) hourly !