Use nightjet.com (no booking fee). The train is probably full a week in advance though.

You won't be able to board without a reservation.

I'll have a look at alternatives.

Thanks for the reply.

Is it possible to book at the train station before boarding?

And I'm a bit worried because you can't reserve any train, that is, there are very few possibilities and they are all full.

We start on August 2 in Milan and we have to arrive in Warsaw on the 7th because we are taking a plane. I am very concerned that there is no room on any train.

Not possible at the train station if the train is full.

Using daytime trains won't be an issue since they don't require a seat reservation (except in Poland but they only cost 0.10€ and you can always get them).

However night trains may be more problematic since they're very popular and have limited capacity + it's a peak season.

Did you have stops en route in mind ?

EDIT : There are seats, couchettes and sleepers on the night train to Vienna leaving at 21:26 on 2nd August from Milan Rogoredo. I'd recommend taking it, book ASAP on nightjet.com.

From Vienna you could take a daytime train to Prague and from there the night train to Warsaw (book on cd.cz). Another route would be using daytime trains to Krakow and then Warsaw.

Let me know for questions.

On nightjet.com, add the Interrail/Eurail discount to see the pass prices. Don't wait to book. I'm surprised there is still availability.

Thank you.

It won't let me reserve any. The one who comments from Milan to Vienna has a very high price.
We had planned to make some stop before arriving in Warsaw, but I see that I cannot reserve any. So we'll take day trains. I have seen that on the interrail page it says that perhaps you can book at the train station.

What about the train that the the reservation is not compulsory but is recommended. could we take this train without reservation? because I can´t reserve these ones.

Did you add Interrail as a discount on nightjet.com ? A seat on the night train to Vienna costs 13.50€ and a couchette 32.80€. Is that what you found ?

Which website are you using for seat reservations ? Rail Planner cannot book seat reservations and only redirects you elsewhere. interrail.eu is somewhat unreliable.

If you have questions, mention route and travel date and we'll gladly give advice on where to book. It is difficult to give generic advice. Please read these links :

https://www.seat61.com/interrail-and-eurail-reservations.htm

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

What does "It won't let me reserve any” mean? Is there an error? Then please post a screenshot. Or else a screenshot of where you get stuck.

"It does not work” is very hard to troubleshoot. I can still book 2 couchette places:

The Interrail website is the last place you should try for reservations. It often has unexplainable errors and is often more expensive.

Without details about route, date and departure time, it's not possible to advise.

Ah okay, I hadn´t added the Interrail discount. Thank you so much for your help. I’ve already got it. 

My final question is about the train from Budapest to Krakow on 6 August at 8:12 am. I can only reserve on interrail.eu website but it said me delivered by post and I don’t understand what that’s means. 

It means that Interrail will mail paper reservations (with a hefty surcharge). Don't do that.

Book on cd.cz : 73 CZK (3€).

That means you're not only paying Interrail's €2 booking fee, but also their €9 postage fee for the privilege of waiting for it to arrive via the post.

You can book this reservation on tickets.oebb.at for €3; use "Seat reservation only” for this train.

Thank you very much for the help. It is helping me a lot to organize the trip.

Thank you!!