I want to associate my Cuore Rossonero Card code with my profile, but I get the error "Invalid Cuore Rossonero code and date of birth. Check your details": what does that mean?

In this case, your Cuore Rossonero Card data and that on your MyMilan profile aren’t the same: it's necessary that the personal data of your MyMilan profile is exactly the same as that shown on the card. If not, update them, save the changes and try again.