Java 20: Unable to declare a global variable with "static"

Question

I'm trying to follow a YouTube tutorial on creating a TicTacToe game.

I'm following along, and the instructions and I'm trying to create the playerPositions and computerPositions as ArrayLists. These need to be public/global/static so methods can read them. But if I try to add "public", "static", "protected", etc in front of the ArrayList, it provides an error and suggests I remove the "public", etc.

I think this is an error specifically for Java 17 to Java 20. Other versions don't seem to have this error.

I've indicated where the ArrayLists are declared and where they're used.

package youtube.alexlee;

import java.util.*;

public class TicTacToe {
    public static void main(String[] args) {
        
        // ***** ⬇️⬇️⬇️⬇️
        ArrayList<Integer> playerPositions = new ArrayList<Integer>();
        ArrayList<Integer> computerPositions = new ArrayList<Integer>();

        char[][] gameBoard = {
                {'1', '|', '2', '|', '3' },
                {'-', '+', '-', '+', '-' },
                {'4', '|', '5', '|', '6' },
                {'-', '+', '-', '+', '-' },
                {'7', '|', '8', '|', '9' }
        };

        while (true) {
            Scanner scan = new Scanner(System.in);
            System.out.println("Enter number (1-9) to place marker.");
            int playerPos = scan.nextInt();
            System.out.println("You selected: " + playerPos);
            placePiece(gameBoard, playerPos, "player");

            Random rand = new Random();
            int compPos = rand.nextInt(9) + 1;
            placePiece(gameBoard, compPos, "computer");
            System.out.println("Computer selected: " + compPos);

            printGameBoard(gameBoard);

            String winning = checkWinner();
            System.out.println(winning);
            if (winning.equals("Congratulations, you win!") || winning.equals("Sorry! Computer wins!")){
                System.exit(0);
            }
        }
    }

    private static void placePiece(char[][] gameBoard, int pos, String user) {

        char symbol = ' ';

        if (user.equals("player")) {
            symbol = 'X';
            // ***** ⬇️⬇️⬇️⬇️
            playerPositions.add(pos);
        } else if (user.equals("computer")) {
            symbol = 'O';
            // ***** ⬇️⬇️⬇️⬇️
            computerPositions.add(pos);
        }

        switch (pos) {
            case 1 -> gameBoard[0][0] = symbol;
            case 2 -> gameBoard[0][2] = symbol;
            case 3 -> gameBoard[0][4] = symbol;
            case 4 -> gameBoard[2][0] = symbol;
            case 5 -> gameBoard[2][2] = symbol;
            case 6 -> gameBoard[2][4] = symbol;
            case 7 -> gameBoard[4][0] = symbol;
            case 8 -> gameBoard[4][2] = symbol;
            case 9 -> gameBoard[4][4] = symbol;
        }
    }

    private static void printGameBoard(char[][] gameBoard) {
        for (char[] row : gameBoard) {
            for (char c : row) {
                System.out.print(c);
            }
            System.out.println();
        }
    }

    public static String checkWinner() {
        List<Integer> topRow = Arrays.asList(1, 2, 3);
        List<Integer> middleRow = Arrays.asList(4, 5, 6);
        List<Integer> bottomRow = Arrays.asList(7, 8, 9);

        List<Integer> leftCol = Arrays.asList(1, 4, 7);
        List<Integer> middleCol = Arrays.asList(2, 5, 8);
        List<Integer> rightCol = Arrays.asList(3, 6, 9);

        List<Integer> cross1 = Arrays.asList(1, 5, 9);
        List<Integer> cross2 = Arrays.asList(7, 5, 3);

        List<List> winningConditions = new ArrayList<List>();
        winningConditions.add(topRow);
        winningConditions.add(middleRow);
        winningConditions.add(bottomRow);

        winningConditions.add(leftCol);
        winningConditions.add(middleCol);
        winningConditions.add(rightCol);

        winningConditions.add(cross1);
        winningConditions.add(cross2);

        for (List l : winningConditions){
            // ***** ⬇️⬇️⬇️⬇️
            if(playerPositions.containsAll(l)){
                return "Congratulations, you win!";
            }
            // ***** ⬇️⬇️⬇️⬇️
            else if(computerPositions.contains(l)){
                return "Sorry! Computer wins!";
            }
            // ***** ⬇️⬇️⬇️⬇️
            else if(playerPositions.size() + computerPositions.size() == 9){
                return "It's a tie!";
            }
        }

        return "";
    }
}

I expected to use static as (e.g.) static ArrayList<Integer> playerPositions = new ArrayList<Integer>(); but it results in an error. I also tried public static and protected static among other combinations.