It would help if you had posted the text of the error message, but it looks like the problem is that you have defined your functions in a class called Driver, but there is no Driver object in your main function.
Try this
int main()
{
Driver driver;
vector<Customer> customers;
cout << "________________________" << endl;
cout << "//MAIN MENU " << endl;
cout << "||Customers (1) " << endl;
cout << "||";
int mainMenuChoice;
cin >> mainMenuChoice;
if (mainMenuChoice == 1)
{
driver.customerMenu(customers);
}
}