I’m trying to add my card to my phone but it doesn’t work

This document explains how to add a payment method to your Apple ID on a Mac, Apple mobile devices, PC, or online --> Add a payment method to your Apple ID - Apple Support

On an iPhone or iPad:

1. Open the Settings app.

2. Tap your name.

3. Tap Media & Purchases, then tap View Account. You might be asked to sign in with your Apple ID.

4. Tap Manage Payments.

5. Tap Add Payment Method.

6. Enter the payment method details, then tap Done.

If something isn't working then you need to provide information that helps us help you such as error messages or an explanation of what you are doing and where it fails to work.