Где я ошибся при попытке вывода сообщения об ошибке?

#include <iostream>
 
using namespace std;
 
int main()
{
    int value;
 
    cout << "Enter number: ";
    cin >> value;
 
    if (value < 0)
    {
        cout << "This is a negative number.";
    }
    else if (value == 0)
    {
        cout << "This is 0.";
    }
    else if (10 >> value >> 0)
    {
        cout << "This number is positive but less than 10.";
    }
    else if (20 >> value >> 10)
    {
        cout << "This number is positive and more than 10 but less than 20.";
    }
    else if (value >> 20)
    {
        cout << "You've entered a positive number, the number is more than 20.";
    }
    else if (value == 10 or 20)
    {
        cout << "Your number is 10 or 20.";
    }
    else
    {
        cout << "You have entered letters not a number.";
    }
 
    return 0;
}

else
    {
        cout << "You have entered letters not a number.";
    }

#include <iostream>
#include <conio.h>
 
using namespace std;
 
int main()
{
    setlocale(LC_ALL, "Russian");
    int x;
    while(x = getch())
    {
        switch(x)
        {
            case 75: cout << "Влево" << endl;
            break;
            case 77: cout << "Вправо" << endl;
            break;
            case 72: cout << "Вверх" << endl;
            break;
            case 80: cout << "Вниз" << endl;
            break;
        }
    }
    return 0;
}

#include <iostream>
 
using namespace std;
 
int main()
{
    int value;
 
    cout << "Enter number: ";
    
    if (!(cin >> value)) {
        cout << "You have entered letters not a number.";
    }
    
    else if (value < 0)
    {
        cout << "This is a negative number.";
    }
    else if (value == 0)
    {
        cout << "This is 0.";
    }
    else if (value > 0 && value < 10)
    {
        cout << "This number is positive but less than 10.";
    }
    else if (value >= 10 && value <= 20)
    {
        cout << "This number is positive and more than 10 but less than 20.";
    }
    else if (value > 20)
    {
        cout << "You've entered a positive number, the number is more than 20.";
    }
    else if (value == 10 || value == 20)
    {
        cout << "Your number is 10 or 20.";
    }
    
 
    return 0;
}

if (!(cin >> value)) {
        cout << "You have entered letters not a number.";
    }