Co mam zrobić jeśli w InteliJ IDEA..

IntelliJ should give you all over the place possibilities to add a SDK. If not... ctrl-alt-shift s then SDKs, click the + (add new SDK) and select 'download JDK' (for CG I'd suggest version 1.8 and vendor Eclipse Temurin) It may be that you need to select the just downloaded SDK for your project. Then again ctrl-alt-shift s - project and here you select as SDK temurin-1.8

Dziękuję za pomoc, ale to chyba nie wszystko. Jeden błąd zniknął, ale ten nadal został: Error: Could not find or load main class pl.codegym.task.task06.task0607.Kot. Wiesz co moglabym zrobic, by go naprawić?

As already said, without knowing your code I just can guess. And my best bet is that the signature of your main method is wrong.

Mój kod

It says you still haven't selected an SDK for the codegym project. That's that part from my explanation above:

It may be that you need to select the just downloaded SDK for your project. Then again 
ctrl-alt-shift s - project and here you select as SDK temurin-1.8

Zrobiłam tak jak pokazałeś, niestety dalej program nie chce się włączyć . Gdy chce zmienić cos w programie pojawia się taki komunikat

This does not look like a CodegymProject. You probably tried to import Codegym files in some weird way into some other project (as a lib or smth like that and now you try to edit the lib??)? If so then this did not work and you either copy the files into the package inside your project or you create the classes there and copy the content into that created classes (adapt the package name). If you intended to work with the codegym project then download it and open it as project (right clicking that folder and select open as project)