I Cloud storage payment

it is not clear why?

On your iPhone, iPad, or iPod touch

1. Go to Settings > [your name] > iCloud > Manage Storage or iCloud Storage. If you're using iOS 10.2 or earlier, go to Settings > iCloud > Storage.
2. Tap Buy More Storage or Change Storage Plan.
3. Choose a plan.
4. Tap Buy and follow the onscreen instructions.

Buy more iCloud storage - Apple Support

https://support.apple.com/en-us/HT201318

To make purchases from the App Store, iTunes Store, or Apple Books, or buy iCloud storage, you need an Apple ID and a valid payment method.

You can typically use these methods with your Apple ID:

• Apple Pay (where available)
• Most credit and debit cards
• Store credit, from redeeming gift cards or from adding funds to your Apple ID
• Country-specific or region-specific additional methods

If you can't make purchases or update apps, including free apps, you need to update your payment method or add a new one. If you already verified that a payment method is available in your country or region and you're still having trouble, learn more.

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

https://support.apple.com/en-us/HT202631

What happens when you try? Any error messages?