maybe the train is already sold out :/

On Thalys (HighSpeedservice on that route) exist just limited seats for Railpassholders and the reservation is quite expensive (20€-30€) :/

I prefer the slower reservation free Intercity´s on that route

If you still wanna use the fast Thalys service you could try to reserve via b.europe https://www.b-europe.com/EN/Booking/Pass#Outbound

For which date is this journey? A reservation can be bought via the Interrail website (and in some other places), if they're still available. What do you mean when you say "I can’t get an option to buy a ticket”? What do you see?

By the way, it is a Thalys train and the reservation will cost 20€ (+2€ booking fee if via Interrail) if available. You can also take one of the hourly Intercity trains without any reservation, just with your Interrail pass. That will take a bit longer but it saves money.

The date would be 4th April, in the photo that I have attached you can see options to “purchase tickets” which I would go with but I then can’t put that journey into the Interrail app because it won’t show up when I type it in. 

is there a website I can visit for the intercity trains? Just so I can fill it out on the interail pass 
 

I see the 9:52 train from Brussels to Amsterdam in the rail planner app.

They’re also included in the rail planner app but the Deutsche Bahn planner (or DB Navigator app) is more reliable and more up to date. The ČD app should be fine too.

If you're not seeing the intercity train you want (e.g. the 9:45 departure is not showing up because it is overtaken by the 9:52 Thalys), go to the rail planner app options and select "No seat reservations required”. That will hide the Thalys trains.

You can add not showed trains in the Interrail app manually by pressing the blue + at the bottom. You can find these in the Railplanner by clicking the “Non reservation” option or search them via bahn.de (or the app DB Navigator) deselect ICE in the details and you will get the hourly Intercity connections starting at 5.44am every hour till 8.44pm :)

I appreciate all the help!!! Thank you so much. I managed to find a longer route but it’ll save the €20. Thank you! 

After re-reading I see you were trying to buy a ticket/reservation via the ČD app. I think reservations for Thalys are not available in the ČD app (hence no link to buy) and possibly ČD doesn't sell them at all.

Reservations for the 9:52 train are still available, e.g. from the Interrail website, but I'd just go with one of the intercity services. In addition, the IC has 3 stops in Brussels (Zuid/Midi, Centraal/Central, Noord/Nord) and one of those may be closer to where you're staying.