I can’t add my card on my phone,please who can help me out?

Try a different card (credit cards are more likely to work than debit cards ; not sure if this still applies in Nigeria : https://discussions.apple.com/thread/4098108?start=15&tstart=0)

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).