Seat reservation on Thalys from Amsterdam to Rotterdam

I was trying to make seat reservation for the Thalys train from Amsterdam to Rotterdam, but when I try to book it, the website gave me the following info:

"These Are Train Tickets.
We do not have any reservations available for this train. You can either inquire locally about the reservation or book a separate ticket."

What does this message entail? Do i need to book a separate ticket for this train?
(I am booking with a Eurail Global Pass.)

Thank you very much.
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