You’re trying to book via Eurail itself? It has a lot of issues lately. You’re not the first.

Check this topic below, and all the posts underneath the main one to see how you can reserve directly with the rail companies:

Which trains and connections you’re looking for? When are you planning to travel? Without details we cannot help.

As you are from Thailand I recommend you to check that your calendar setting is the greorian calendar. This is also important if you will be using the mobile pass. 

As @BrendanDB says there are lots of problems with reservations through the Eurail website. If you try to make too many reservations at the same time the system does not manage.  It is better to make reservations elsewhere, see the link in the reply above. 

If you let us know what reservation you want to make you will get good advice here.

Watdee khrap. Kamlang yoo tee PraNakorn

Pai is a lovely small village north of ChiangMai.

Yes-it is for the app etc-and also very much when you activate, year 2022, not 2565.

And as anyone above already explains:  always better to use other ways to make these RES-and if you can tell us for what trips-there may also be alternatives that use other trains and are without, but may take longer.

@BrendanDB Thank you for your help. our travel plan is as follows : 

23 Dec 2022 : Paris => Strasbourg   <We expect to travel by TGV> 

26 Dec 2022 : Strasbourg => Paris <We expect to travel by TGV> 

our problem found when we reserved the seat via website is at the final process of payment.

It cannot pay and go back to the start to seat reservation window again. 

Anyway, do we need to reserve the seat before? or we can directly buy at the station. 

<We we have already reserve the ticket by Rail Planner mobile apps.> 

Best regards, 

Pai 

Many thanks @AnnaB, Our trip as bellow: 

23 Dec 2022 : Paris => Strasbourg   <We expect to travel by TGV> 

26 Dec 2022 : Strasbourg => Paris <We expect to travel by TGV> 

<We we have already reserved the ticket by Rail Planner mobile apps but cannot reserve the seatss>

Best regards, 

Pai 

@mcadv Sawasdee khrap, now you stay in Bangkok? If yes, May I go to meet you? I really need your help. anyway for our trip as bellow: 

23 Dec 2022 : Paris => Strasbourg   <We expect to travel by TGV> 

26 Dec 2022 : Strasbourg => Paris <We expect to travel by TGV> 

<We we have already reserved the ticket by Rail Planner mobile apps but cannot reserve the seatss>

Best regards, 

Pai 

There's no need to activate your pass and to add your journey in the app until shortly before your journey. If there are mandatory reservations, like in this case, you should always book those first before activating a travel day.

To book these TGVs, you can use this page. Book them as a return, that will be cheaper on this page because you'll pay the booking fee only once. To book on this page, you'll need to enter a pass cover number, which you can generate here from your mobile pass number.

Many TGVs on these dates are already full and in most remaining ones, only 1st class is still available. In France however, if you have a 2nd class pass, you can also book a 1st class reservation. It's just a bit more expensive.

Do you have a 2nd class pass or a 1st class pass? How many travellers are you?