Hangman Program - No Suitable Method Found Error

I am trying to create a Hangman program using Arrays. When trying to compile my code I get the following error: 50: error: no suitable method found

for setCharAt(int,String)
method StringBuffer.setCharAt(int,char) is not applicable
method AbstractStringBuilder.setCharAt(int,char) is not applicable

Here's the code:

import java.util.*;
import java.io.*;
public class hangman {
public static void main() throws IOException {
Scanner kb = new Scanner(System.in);
String filename;

[code]....