Which train your trying to reserve and when?

If it's Thalys, don't take it. It's to pricey for such a relatively short journey.

Just take the international hourly IC from Amsterdam to Brussels, doesn't need any reservation. 

Are you sure I can take the train when it’s says the reserve is required? Because I send an email to Eurail and they told me it’s was necessary to reserve, however I couldn’t take the train. 

You didn't specify here which train: train number or departure time.

But when it says reservation is required, then you need one.

However, there are also reservation-free intercity trains Amsterdam-Brussels each hour and you can always use those.

My train is: July 3 2023 8:15-10:08. THA 9322. Amsterdam Centraal - Bruxelles MIDI

As said above, Thalys trains require a very expensive seat reservation (27€). Really not worth the cost. That train is sold out in 2nd class tomorrow anyway. Do not try to board or you'll get a big fine.

Instead take the hourly reservation-free IC train :

- 06:28 - 09:17

- 07:28 - 10:17

- etc.

Do you have an outward connection ?