Are you trying to buy a full ticket, or travel with a railpass?

In either case, I would break the journey down, and book Wroclaw-Warsaw as one journey, and Warsaw-Vilnius separately.

I’m not can buy full ticket, railpass same problem

Railpass

You can buy a reservation on the sleeper from Wrocław to Warsaw at https://www.cd.cz

Reservation from Warsaw to Vilnius can be bought from the ticket office at Warsaw

Full Tickets

Buy at www.intercity.pl 

Since you seem to live in Poland, it's best to go to a PKP IC ticket office.

Lithuania, Latvia, Estonia only in ticket office? It feels like we live in the USSR xD

By the way, I noticed that tickets on the Czech website are cheaper than from the intercity, very interesting

To book a night train, the Czech website also asks you to write an email or call

I think sometimes it works online, sometimes not! 

If you’re in Poland, I would visit a ticket office to see if you can buy a reservation for the sleeper.

Or if the sleeper is not available, you can take the day train to Warsaw or Białystock, spend the night, and continue to Vilnius.

Lithuania, Latvia, Estonia only in ticket office? It feels like we live in the USSR xD

It’s only for the railpass reservations. They’re always difficult, everywhere. When I bought a ticket Kaunas-Białystock from LTG, I could do it online and add to my Apple Wallet!

For a seat in 2nd class with an Interrail ticket 1 zloty on the Intercity website, that’s already good, but I would like a sleeper

It works for this train if you add Interrail as a discount.

In Estonia, last time I was there (2019), they still had chipcards that you could use. You could either buy a ticket in advance or from the ticket inspector.

In Latvia, in the same year, we just bought our tickets from the ticket office. There weren't any reservations and the price was fixed, so no need to buy in advance.

Pass reservations for Warsaw to Vilnius and Vilnius to Riga can be ordered from Lithuanian railways by e-mail.