I can’t able to add any card in my wallet? What to do?

What happens when you try? Do you get an error? If so, what does it say?

If you get the Learn More popup that takes you to the support document you posted from when tapping on the (➕) icon in the Wallet app, then make sure your iPhone is set to a country or region where Apple Pay is supported in Settings➜General➜Language & Region➜Region. click here ➜ Countries and regions that support Apple Pay - Apple Support

You're welcome. Glad it worked out.