You need to import junit.Assert if you want to use assert in Java. You also need to fix up the actual assert line to use equals since we are doing a string comparison. Change the default test case to the following:

import org.junit.Test;

import static junit.framework.TestCase.fail;
import static org.junit.Assert.*;

public class Tests {
  @Test
  public void Test1() {
    assertEquals(Kata.HighAndLow("8 3 -5 42 -1 0 0 -9 4 7 4 -4"), "42 -9");
  }
}