I can not add my payment method

Re: I can not add my payment method : I can not add my Visa card to my Apple family sharing plan.

Wondering if you are the family organiser, and whether you want to turn on purchase sharing using your iPhone

What is purchase sharing?

Purchase sharing is a feature that you can use after you set up Family Sharing. When you turn on purchase sharing, everyone in your family gets access to the apps, music, films, TV shows and books that family members buy from Apple.

Learn how shared purchases work with Family Sharing,

what the family organiser is responsible for,

and how to update your payment methods:

or

If you're the family organizer and you want family members to pay for purchases themselves, turn off purchase sharing: 

1. If you haven't set up Family Sharing yet, set up Family Sharing.
2. Open the Settings app.
3. Tap Family. If you don't see Family, tap your name, then tap Family Sharing.
4. Tap Purchase Sharing.
5. Tap Continue and follow the onscreen instructions.
6. To confirm which payment method will be billed, tap Purchase Sharing again and check the Shared Payment Method information. This will be the family organizer's default payment method. 

All above is from: How to share apps and purchases with your family - Apple Support

If you do use purchase sharing, you can download apps and play content that your family members buy.

Also: How to download apps and content that your family members purchased - Apple Support

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Re: I need to get it added so it doesn’t cancel or mess up our plan.

If everyone in family sharing pays for their own purchases:

Find your country or region to see what payment methods are available

Payment methods that you can use with your Apple ID - Apple Support

How to Add an Apple ID Payment method:

Add a payment method to your Apple ID - Apple Support

You may need to allow 24 hours for updates to come into effect.