Your best bet is to contact Eurail support as they may be able to add a day if you have a digital pass. What happened to you is why you must NEVER activate a day until just before you board the first train

Yeah normally I don‘t activate it before, but i needed to scan the ticket, to get to the platform..

I did, they told us to send a picture that the train was really cancelled… and I can‘t find it anywhere.

I don’t know how you could get that info but Eurail should have a contact in SNCF as they have a database of all delays and cancellations on the network. In UK this information can be found in a few clicks. Anyone know if there is a portal for SNCF? Otherwise put the ball in Eurail’s court and ask them what you can possibly do to get the iinfo they want. I’d point out you never activate a pass day in advance but you had to access the platforms and SNCF information is often late to non existent. Good luck and don’t give up!