Q:

why i'm getting void type not allowed

when I'm trying to print the method I get an error on second line saying
void type not allowed here
 public static void main(String[] args) {
System.out.println(printMenu());
  }

 public static void printMenu(){

 System.out.println("***** Welcome to KAU Flight Reservation System *****"+
        "\n 1. Add Flight Details in the System"+
        "\n 2. Add Passenger Details in the System"+
        "\n 3. Make a new Booking"+
        "\n 4. Search and Print a Booking"+
        "\n 5. List Flight Status"+
        "\n 6. Exit from the System ");

}

A:

there are two kinds of functions in java. Procedures which They don't return any value and Functions which  return value. when you adding the word void in the method declaration it means that this is a Procedures method and it doesn't return any values however the System.out.print() method require a parameter of Object kind.
your printMenu() method should be like this. 
 public static String printMenu(){

 String menu = "***** Welcome to KAU Flight Reservation System *****"+
        "\n 1. Add Flight Details in the System"+
        "\n 2. Add Passenger Details in the System"+
        "\n 3. Make a new Booking"+
        "\n 4. Search and Print a Booking"+
        "\n 5. List Flight Status"+
        "\n 6. Exit from the System ";
return menu;

}

