What model device are you using? What iOS version is the device using? What country are you in? What country is the bank in that issued the card? Is the device issued to you by a corporate entity, or in other words, is it managed by a corporate IT team?
Apple does not approve or decline adding a card to your Wallet app. Your iPhone sends encrypted information about your iPhone and the information you entered when attempting to add your card, to the Payment Network Operator (Visa, MasterCard, American Express and Discover) and if they approve, it’s sent to the issuing bank for final approval. Information you’ve entered on your iPhone, not entered or entered incorrectly could look like fraud to the PNO or your bank. If the algorithm detects suspected fraud, your request is rejected and an appropriate error notification is displayed in the Apple Wallet app.