Well, the problem clearly is Java can't find class MorseCodeDecoder that you should have implemented in your solution like this:

public class MorseCodeDecoder {
    public static String decode(String morseCode) {
        // your brilliant code here, remember that you can access the preloaded Morse code table through MorseCode.get(code)
    }
}

Are you trying to run your tests here on CodeWars, or on your computer?