Calculator v1

[Brainless Control]

Here I don't have the time to debug your code, but I have an modification to offer you ;)
Compiled and works as you wanted it to!

Code:

#include <iostream>
using namespace std;

int option = 0;
double a = 0;
double b = 0;

int showMenu()
{
    cout << ":::Calculator v1::: \n";
    
    cout << "Enter a Number: \n";
    cin >> a;
    cout << "Enter a Number: \n";
    cin >> b;
    cout << "1. Multiplication \n";
    cout << "2. Division \n";
    cout << "3. Addition \n";
    cout << "4. Subtraction \n";
    cout << "5. Enter new numbers \n";
    cout << "6. Quit \n";
    cout << "Enter your choice: ";
    cin >> option;  
    
    return option;
}

void Calc(int option, double num1, double num2) {
    a = num1;
    b = num2;
    switch (option) {
        case 1:
             cout << "The Result is: " << a*b << '\n';  
             break;
        case 2:
             cout << "The Result is: " << a/b << '\n';
             break;
        case 3:
             cout << "The Result is: " << a+b << '\n';
             break;
        case 4:
             cout << "The Result is: " << a-b << '\n';  
             break;
        case 5:
             break;
        case 6:
             cout << "Goodbye \n";
             break;
    }
}

int main()
{
    do {
        option = showMenu();
        do {
           Calc(option, a, b);
        } while(option<1 || option>6);
    } while(option!=6);
        return 0;
}

or that lol,very smart move