Why can't I add my Debit card to my apple pay

Why not? 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, 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