Why can’t I add my card to my Apple Pay.

Your question provides very few clues as to why it is not working and how you have discovered that a card is not being added so it is difficult to answer.

Read carefully the instruction in Set up Apple Pay with particular attention to:

Countries and regions that support Apple Pay - Apple Support

and

Apple Pay participating banks in Canada, Latin America, and the United States

Apple Pay participating banks in Africa, Europe and the Middle East

Apple Pay participating banks and card issuers in Asia-Pacific

If this does not help you contact Apple Support via the link at the foot of the page:

Apple Pay - Official Apple Support