Help Needed With "Unique Characters in a String" Interview Question

Link to question:
https://www.codecademy.com/code-challenges/code-challenge-unique-characters-in-a-string-java

My code:

import java.util.*;

public class UniqueCharacters {
  public static void main(String[] args) {
    System.out.println(uniqueCharacters(""));
  }

  public static boolean uniqueCharacters(String strIn) {
    if (strIn.length() == 0) {
      System.out.println("Error");
      return true;
    }
    ArrayList<String> list = new ArrayList<String>();
    for (int i = 0; i < strIn.length(); i++) {
      if (list.contains("" + strIn.charAt(i))) {
        return false;
      }
      list.add("" + strIn.charAt(i));
    }
    return true;
  }
}

This consistently gives a result of 4/5 tests passed. I believe the issue is with this part of the question:

Note that if the function is called with an empty string (“”), the program should return an error message.

However, I don’t know how to fix this. If you try to change the method’s return type to something like Object so you can pass an error message or boolean as the result, the tester breaks. If you try to throw an exception, the tester breaks. I don’t know how else to pass an error message, and neither making the method return either true or false when the input is a blank string works. Any help is greatly appreciated!