Is earning from selling WhatsApp codes permissible or does it involve a doubt, knowing that the uses of these codes are unknown, and it is said that they might be for advertisements?

If it has a permissible benefit, and no haram or assistance to haram results from it, then there is no objection to selling it and benefiting from it. However, if it is known that the intention of those who want these codes is to use them for something haram, then it is not permissible to sell them to them.