Unfortunately Eurostar limits the number of seats for pass users. The advice is to click through until you find an available train. Alternatively you can consider one of the alternative stations Paris, Lille or Brussels.

I'm guessing you need to travel from London to France. 29 September is quite soon for Eurostar reservations, so book ASAP.

Availability London - Paris currently looks like this (only 2 trains):

And London - Lille like this (also just 2 trains, or just 1 in 2nd class):

London - Brussels:

If you look at the official Eurostar website you will see seats available until all seats on the departure are sold. However, the pass holder seats for Interrail/Eurail sell out before the departure is sold out. 

You can check the availability of pass holder seats and make the reservation here:

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

If you have a mobile pass you need a Pass Cover Number to make the reservation. You can generate your PCN here:

https://community.eurail.com/news-and-announcements-39/pass-cover-number-generator-is-live-5653

Thank you very much for your answer guys ! I changed all my ferries and trains to make it match the available eurostar. I didn't know about this limit for pass holders.