Поменяй /n на endl cout << "Menu:\n" 
<< "1. Add a new car\n" 
<< "2. Print the information about a car\n" 
<< "3. Search for cars that need maintenance\n" 
<< "4. Search for cars of a given brand\n" 
<< "5. Search for owners with mileage greater than a specified one\n" 
<< "6. Exit\n"