Can’t add card or apply card on my iPhone why

Hi winful.

Welcome to the Apple Community.

You might need to check if Apple Pay is available in your country or region:

and 

you can follow the link in the article to see which card providers Apple accepts in your country or region.

Countries and regions that support Apple Pay – Apple Support

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NB: The "Related Article" above your question is for store cards etc, not for credit / debit cards.

To add a debit or credit card:to Wallet to use with Apple Pay:

Set up Apple Pay

says:

In the Wallet app, tap the Add button (+).

✽. Tap Debit or Credit Card to add a new card.

✽. Tap Continue.

✽. Follow the steps on the screen to add a new card.

✽. Verify your information with your bank or card issuer

They might ask you to provide more information,

or to download an app before approving your card for use with Apple Pay.

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If you still need help:

What to do: If you can't add a card to Apple Wallet to use with Apple Pay - Apple Support

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If needed:

How to apply for Apple Card - Apple Support

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For support if needed:

Apple Pay - Official Apple Support

Apple Card - Official Apple Support

or

See a list of Apple phone numbers around the world

Contact Apple for support and service – Apple Support

All the best :-)