why does it decline my card when i try to add it when i made sure my card is perfectly fine to use

It could be due to a number of reasons e.g. it no longer satisfies iTunes validation (validation rules aren't fixed, it can change over time ; debit cards seem to be accepted less than they used to be), or the card issuer might be declining it for use with iTunes. Do you have another card that you could try ? Or have you tried just logging into the account (not when trying to download something) and seeing if you get the 'none' option so that you can remove the card (that would then let you download app updates) : Change or remove your Apple ID payment information - Apple Support (if you do get the 'none' option you could also try re-entering the card's details from scratch and see if it's then accepted)

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).