why task isn't finished

The konwertujCelsjuszaNaFahrenheita() method, according to the task, needs to convert any integer passed to it to Fahrenheit. The code you shared has the method only returning the calculation 9 /5.0 * 41 + 32. Since it doesn't work the way it is supposed to, the validation system fails it. You are going to need to fix this to pass. Put this code in your main method:

public static void main(String[] args) {
   System.out.println(konwertujCelsjuszaNaFahrenheita(41));
   System.out.println(konwertujCelsjuszaNaFahrenheita(22));
   System.out.println(konwertujCelsjuszaNaFahrenheita(36));
}

The correct output for this, when your konwertujCelsjuszaNaFahrenheita() method is coded correctly will be: 105.8 71.6 96.8