Ok, I see that someone has also had this problem, and I saw the “remedy” of this issue by the tiny disclaimer at the bottom of the reservation timetable. I must have read every part of the EURail site but those 2 lines! 

This is a known problem and unfortunately Interrail/Eurail seem not to want to do anything about it. The experienced travellers in the community have raised the issue to the Eurail team on several occasions. 

There are more topics on the subject in the Community. Here is one:

https://community.eurail.com/eurail-interrail-passes-41/eurostar-problems-booking-seat-reservation-using-interrail-pass-6728

You can relatively easy avoid travelling with Thalys, should there be no seats available. There are other trains you can take, but a tad bit slower maybe.

Ordinary TGV have cheaper reservations (and better passholder quota) and IC-trains, often don’t have no obligatory reservation.

When and where you want to go?  We could help you find alternatives.

Hello! Thank you for your question. Unfortunately, our Reservation Service does not have insight on seat availability without trying to book a reservation. You could try the website of the corresponding carrier and check if there are seats available, but I’m afraid that information is very fragmented, and don’t (correction: don't always) show that some trains have limited seats available for Pass Holders. For some trains like Eurostar, ICE, and TGV it is highly recommended for customers to book in advance, especially when traveling during the high season (Mid-March to October). 
We also mention on our website, that whenever the Pass holder's seats are sold out, the customer can either book an alternative train or purchase a full fare point-to-point ticket. I understand it can be problematic for those on a tight schedule, but this is why we let and encourage our customers to book 3-6 months in advance.

If your Pass is not activated and used, you can apply for a refund or travel at a later time when the desired trains are available to be reserved ! :)

I hope this clarifies it! Please let me know if you have any additional questions.

Kind regards,

Annie

@Annie.

Yes, we know the real availability check is only when you try a real booking, but as I have explained before, that is by no means a valid excuse to hide from your potential customers the search results you get after having bought a pass. The reservations module does (or should) show for which trains it is sure reservations are not available anymore and that is already a valuable piece of information. Hiding that  information only results in frustrated and angry travellers, as we have seen on many occasions.

That also means that this topic is not "solved”.

For Eurostar and Thalys there is no problem to check pass holder seat availability before buying the pass and it is these trains that cause the most problems for travellers, so please give correct information in an answer that you mark as "best answer" @Annie. 

Here is the link to the Website where you check the pass holder seat availability.

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

@AnnaB This applies in general cases regarding Pass holders seats, not particularly for Thalys and Eurostar. 

@rvdborgt 
While I understand your frustration, please allow me to clarify some points. Our system does not have insight on seat availability without trying to book a reservation and for this one needs to have an account, create a trip and add a valid Pass. The information you see on the timetable does not reflect the availability but shows you that the connection requires an additional seat reservation. The shown fee indicates only the cost of a reservation for a Pass holder.

I would like to emphasize that Eurail.com does not own or operate trains, therefore we cannot control the availability of the seats. Regarding availability, we rely on the data provided to us by the railway companies.

@Annie. 

That is how it is set up now, but it doesn't have to be that way. As I've explained before.

It is possible to show pass holder seat availability without having a trip or a pass. Just assume a valid pass for the journey entered and have a selector for 1st and 2nd class. Belgian railways (and others) do it that way, so any claim that it isn't possible is not credible.

@rvdborgt Since there are several Passes available, both for 1st and for 2nd class, different conditions would apply.  Thank you for your feedback, I will pass this to the relevant team! 

Rvdborgt, AnnaB, thanks for your comments.
When an answer is marked as best answer it will show as solved. The question by Lisey: '’What can I do besides just request refund?'’ is answered by both BrendanDB and Annie. They suggest that you can either book an alternative train or purchase a full fare point-to-point ticket and that we encourage to book well in advance. Another suggestion is to travel at a later moment when the desired train is available. 
AnnaB, you are right so thanks for the correction. I've corrected it in Annie her comment. 

Nevertheless, you are right in that the cause of Lisey situation- that reservation availability is not shown before buying the pass- did not change. Thanks for raising your feedback with us. It's very much appreciated. AnnaB, I have to correct you on your statement that Interrail/Eurail doesn’t want to do anything about it. In fact, the relevant teams have this on their radar and are working on improvements, but before I elaborate, let me give you some background information on this topic.  

It was a long wish for customers to see indication prices to know what costs they would have to take into consideration on top of the pass. This has been implemented 2 years ago in the new timetable and in the app. Now we see that customers are seeing indication prices as real prices and availability and as long as there was enough availability of trains, this has not been an issue. Now, during this high season we’ve seen that many trains were sold out or cancelled and therefore it became an issue for travelers.

An improved design has been created to clarify that the prices shown (before log in) are just indication prices and real availability is only shown when you book. This will be released over the course of next week. There is also a new design ready for the app that clarifies that the prices shown are an indication. The release is planned for mid-November.

This way we at least show more clearly that only indication prices are shown. However, that doesn’t detract from the fact that people still would like to see the real availability before they are buying a pass. Unfortunately, there is no solution for this (yet). There are certain factors that limit us to show real life prices from our booking systems to all visitors planning their trips. Nevertheless, the relevant team continues to look into possible solutions. But to manage expectations, this is not something we will have in the short run. I will keep you informed. 

@Nanja 

Thanks for your answer. It is good that you will make it clearer on the website that real availability only is shown after you have bought a pass. 

As it is possible to see the availability of pass holder seats for the Eurostar and Thalys without a pass at b-europe, I think that it would be good to inform about that possibility on the Interrail/Eurail website, with a link to the correct website of b-europe, so that travellers can check for themselves until you have implemented that functionality. 

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

Thanks for the suggestion, Anna. I will forward it to the relevant teams. Once I know more, I will update you here. 

Many thanks for this. It's the first time someone from Eurail actually says you want to implement this, instead of citing technical reasons for not doing it.