Ok I guess this means the task doesn't have test cases like abc999999999999999999999999999

Yeah I was wrong. It's not BigInteger not working, it's their information about their tests that was weird

import java.math.BigInteger;

public class Kata {
  public static String incrementString(String str) {
    BigInteger n = new BigInteger("1234567890"); 
    return str; // you code here
  }
}

That works just fine. Not a kata issue.

java.math.BigInteger doesn't seem to work on this machine here