local variables referenced from an inner class must be final or effectively final

You keep updating both high and low inside the run() method, making them by definition not effectively final.

Since you don’t need them outside the run() method anyway, just move the two lines inside.

public void HiLo(int[] numbers){

    Runnable r2 = new Runnable(){
        @Override
        public void run() {
            int high = numbers[0];
            int low = numbers[0];
            System.out.println("The highest value is: ");