Why does the other player never gets its turn in my Last Stone Game (Nim) implementation?

package HomePlace;

import java.util.Random;

import java.util.Scanner;

public class LastStone {
	public static boolean isValidEntry(int left, int User) {
		if (left > 0 && User < 4 && left >= User) {
			return true;
		} else {
			System.out.print("Invalid entry!!");
			return false;
		}
	}

	public static int userMove(int left) {
		Scanner input = new Scanner(System.in);
		System.out.print("There are " + left + " stones. How many would you like? ");
		int User = input.nextInt();
		return User;
	}

	public static int computerMove(int left) {
		int cpuStones;
		cpuStones = generateStones();
		System.out.print("There are " + left + "stones. The computer takes" + cpuStones + " stones");
		return cpuStones;
	}

	public static int generateStones() {
		Random sr = new Random();
		int gstones = sr.nextInt(2) + 1;
		return gstones;
	}

	public static void playLastStone(int UserOrCpu) {
		Scanner input = new Scanner(System.in);
		if (UserOrCpu % 2 == 1) {
			System.out.println("The Computer beats the User!");
		} else {
			System.out.println("The User beats the Computer!");
		}
	}

	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);
		Random r = new Random();
		int left = r.nextInt(16) + 15;
		int UserOrCpu = 1, User = 1, temp;
		while (true) {
			if (UserOrCpu % 2 == 1) {
				User = userMove(left);
				if (isValidEntry(left, User)) {
					if (left == User) {
						playLastStone(UserOrCpu);
						break;
					}
					left -= User;
				} else {
					continue;
				}
			} else {
				computerMove(left);
				temp = computerMove(left);
				if (left <= temp) {
					playLastStone(UserOrCpu);
					break;
				} else {
					left -= temp;
				}
			}
		}
		UserOrCpu += 1;
	}
}

My computer player cpu never takes its turn in my implementation of the Last Stone game or Nim.

I found similar questions but none helped with solving my problem.

I debugged my code line by line but couldn't figure out why my second } else { in my main method is never executed?

                    left -= User;
                } else {
                    continue;
                }
            } else { // why is it never executed?

In your given code, your main method actually reduces itself to

    public static void main(String[] args) {
        Random r = new Random();
        int left = r.nextInt(16) + 15;
        int userOrCpu = 1, user;
        while (true) {
            user = userMove(left);
            if (isValidEntry(left, user)) {
                if (left == user) {
                    playLastStone(userOrCpu);
                    break;
                }
                left -= user;
            }
        }
    }

And so it's now clear that your computer player cpu never gets its turn.

The problem is that your UserOrCpu += 1; assignment isn't in the right scope, and so doesn't have any effect.

The "fixed" main method should look like this

    public static void main(String[] args) {
        Random r = new Random();
        int left = r.nextInt(16) + 15;
        int UserOrCpu = 1, User = 1, temp;
        while (true) {
            if (UserOrCpu % 2 == 1) {
                User = userMove(left);
                if (isValidEntry(left, User)) {
                    if (left == User) {
                        playLastStone(UserOrCpu);
                        break;

                    }
                    left -= User;
                } else {
                    continue;
                }
            } else {
                computerMove(left);
                temp = computerMove(left); // executes computer move a 2nd time?
                if (left <= temp) {
                    playLastStone(UserOrCpu);
                    break;

                } else {
                    left -= temp;
                }
            }
            UserOrCpu += 1;
        }
    }

Hint: Get yourself an IDE (like IntelliJ) for writing code. Such a code editor will indicate "obvious" problems with your code immediately.