I’m trying to update my debit card but won’t let me

Does it say why it won't let you?

From: If you need to change or update your Apple ID payment method - Apple Support

"If you change your address, the credit or debit card information that you use with your Apple ID might be automatically updated by your financial institution, and you don't need to take action. But if there's an issue, add a new card first, then remove the old card."