It might be because it's a debit card, not all seem to still be accepted, and might no longer be accepted at all in some countries.
For a card to have a chance of being accepted it needs to be registered to the same name and address (including format and spacing etc) that you have on your iTunes account, and have been issued by a bank in the country where you are (and therefore the country which is on your iTunes account). If it is then you could check with the card issuer to see if it's them that are declining it.
You don't need to put in a new card to download updates to apps (unless you owe money or are the family organiser in family sharing), but if you have a card on your account then it has to be a valid card. Have you tried logging into your account, not when trying to download something, and seeing if you get the 'none' option so that you can remove your existing card : Change or remove your Apple ID payment information - Apple Support ? If you can you should then be able to download updates (and possibly try entering the new card's details from scratch)