Q: Why is my Apple ID saying card declined when the details are correct to register my new card details to my account

Not all debit cards are accepted payment methods. Payment methods you can use in the iTunes Store, App Store, and iBooks Store - Apple Support

Also, generally, when a card is declined, it is based on information from the financial institution that issues the card. Check to make sure the name, account number, address and other information match the billing information exactly. Things like the address are usually the culprit here. The difference between ST and Street, AVE and Avenue, etc.. If necessary, contact iTunes support for assistance. https://www.apple.com/emea/support/itunes/contact.html