non static/ static variable error

The other answers are around changing things to static, you should avoid having mutable state in static variables this is 101 of programming - it's essentially global variables.

import java.util.Scanner;
public class CH6PA
{
private double difficulty;
private int score;
private double[] average;

public static void main(String[]args)
{
    CH6PA ch6pa = new CH6PA();
    ch6pa.doSomething();
}

private void doSomething() {
    Scanner keyboard = new Scanner (System.in);

    do {
        System.out.println("Enter the level of difficulty (1.2-3.8)");
        difficulty = keyboard.nextDouble;
    } while (difficulty>1.2 || difficulty<3.8);

    int judge = new int[7];

    for(int i = 0; i<judge.length; i++) {
    }

Be careful with the do { } while();. Your condition is going to be always true, creating an endless loop:

     while (difficulty>1.2 || difficulty<3.8)

Ex:

    1.0 (false || true ) => true

    4.0 (true || false ) = > true

    2.0 (true || true) => true

Instead, define your condition as:

    while (difficulty<1.2 || difficulty>3.8)

And good luck for the rest!

The problem as the compiler suggested is that you are using the difficulty instance variable in a static context (ie: not as part of an instance). What you need to do, is if you believe difficulty should be the same in all instances of the class CH6PA then I would set the difficulty to static by putting the static keyword before it in the declaration.

The reason you are getting this error is you are referencing the difficulty field in the main method which is declared static. But difficulty is defined as non-static (an instance field). So difficulty is part of a CH6PA instance. But the main method is not. You can only access instance variables from instance methods.

As Reseter said above, your difficulty variable should be declared static because of your static method does not have an access to the non-static property.

You may find complete explanation here: http://download.oracle.com/javase/tutorial/java/javaOO/classvars.html