Please I'm using an iPhone 6s plus and I haven't ever installed my payment details and for the first time it keeps declining it

If it’s a debit card that might be why it’s being declined, not all seem to still be accepted by iTunes. Try a different card (preferably a credit card).

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).