Can’t add new payment card

Start by updating your iPhone 6 to the latest available version: iOS 12.5.7

You can update your iPhone here:

Settings → General → Software Updates

If you want to add your card to Wallet and use it with Apple Pay, please note that not all card types, regions and banks support this. You can add credit, debit, or prepaid cards.

You can see how to set up your card for Apple Pay here:

Set up Apple Pay - Apple Support

Countries and Regions that support Apple Pay:

Countries and regions that support Apple Pay - Apple Support