'Login' identifier not found

You cannot us a function the compiler hasn't seem defined yet. So, you have two options. You can either put the function definition above your main function, like this:

#include "Login.h"
#include <windows.h>
#include <iostream>
using namespace std;

void Login();

int main()
{
    srand(time(NULL));

    Login(); // Login
}

void Login()
{
    ...
}

or move the entire Login function above main:

#include "Login.h"
#include <windows.h>
#include <iostream>
using namespace std;

void Login()
{
    string userName;
    string userPassword;
    int loginAttempt = 0;

    while (loginAttempt < 5)
    {

        std::cout << "Username:";
        std::cin >> userName;
        std::cout << "Password";
        std::cin >> userPassword;

        if (userName == "logintest" && userPassword == "logintest")
        {
            std::cout << "\nLogged in";
        }
        else
        {
            std::cout << "\nError";
        }
    }
}

int main()
{
    srand(time(NULL));

    Login(); // Login
}