Yes it can't be booked on interrail.eu until 90 days prior to travel date. However as it is already possible to book regular tickets on sncf-connect.com you could call SNCF (no booking fee), see :

Thank you. One more thing: Since I don't know how to speak French, I think it's better to wait for the option to book them through the Interrail app. Or can it be late? Do I run the risk of running out of ticket if I wait until I can book them through the INterrail app?

The app can't book any reservations. It just forwards to some of the websites where you can book, including the Eurail website.

The SNCF phone service also has an English option, please check the post mentioned above.

You can also wait. This train does not have limited pass holder seats, so you can book until it is full.

Thanks again. Now I have to book our seat for a night train from Mannheim to Prague.

In this case, the app redirects you to the Interrail website, but once there it doesn't show you how to book either. The app also tells you three websites where to book, from different companies, but once inside these websites there is no way to know how to book with an Interrail pass. In this case I am more concerned because it is a night train and we are traveling with children, so I would like to reserve beds. That I have to do? Do I always have to call by phone?

For this night train use nightjet.com or tickets.oebb.at. There you can add Eurail as a discount

Yes it's a good idea to book night trains well in advance. They easily sell out

Sorry but I stil have doubts. In the DB website and in the tickets.oebb.at website appears like it’s not possible to make seat reservations (screenshots right below this). In the nightjet.com website is possible but with some strange conditions, like we are not allowed to travel in one space together because the maximum are triple bed compartment.

Reservations in this night train don't seem to be open yet for August.

So Does it mean that maybe in May seat reservations will be available? and, maybe, with better information about compartments, beds, possibility of share the same compartment the four members of the family… in the page of DB?

This night train is run by CD, OBB and SBB AFAIK so you’ll find better info there. DB usually isn’t that complete. I’d recommend cd.cz or nightjet.com (when it’s available them)