The likely reason is that it WAS already just ´full´ when you tried to book. Or rather: the small quota they set aside for passholders (TLS).

Avoid Tls=way overpriced and always ALTernative via other trains at lower price. Passholders need to be creative and not blindly follow 1st presented option=usually the highest cost and hardest to get.

But again-without precise details we cannot present yet other ways.

I assume you did via app? In that case: again-we have to tell each&every newbee: use the advice of seewulf on howtodoit on an overview page.

The advice from the experienced travellers in the community is to use other ways to make reservations than the Interrail/Eurail website. You can look at the guide in the link:

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

If you, after having looked at the guide, have questions about how to make specific reservation, please give your travel details (departure date, time and route) preferably in a new post, and you will get advice. 

You need to re-do your travel plan. Either change the dates and travel on days where there are seats available, or change the route and not use the high speed trains that are fully booked.

If you share your travel details you will get advice on alternative routes for your travel.