Cannot Find Method Append Error

I am trying to run the following code but getting the error above.
 
import java.io.*;
public class StringBuilder {
    public StringBuilder() {
        super();
    }   
    public static void main(String args[]) {
            // creates empty builder, capacity 16
            StringBuilder sb = new StringBuilder();
            // adds 9 character string at beginning
            sb.append("Greetings");
    }
}