Add to Apple Wallet button is there but it doesn’t work

You cannot add a Debit/Credit Card to Apple Pay if the county that you have chosen under the Apple ID does not support it. (e.g. India)

Adding Credit/Debit Cards to Apple Wallet depends on the country that you live in --> Countries and regions that support Apple Pay

However, you may add any of the following passes/tickets

Adding Boarding Passes, Bookmyshow Tickets, IRCTC (Railway) tickets, Airline Tickets, Game Tickets and Hotel Bookings are only possible if the issuer has enabled the "Add to Apple Wallet" feature for the tickets/passes issued by them. See some examples below.

The card issuer must enable the "Add to Apple Wallet" feature.

You will find this in the Card Issue Document

1. Tap Add to Apple Wallet.
2. 

Where did this "Add to Wallet" button come from? Email from a pass or ticket provider? In an app? or a website?

If any of the previous 3, contact the provider and tell them their button does not work.

It's up to the provider to code the button so it opens the Wallet app and adds the pass or ticket to it.

Define "doesn't work".

What happens when you try to use it?

Do you get an error? If so what does it say?

You could have it disabled in screen time