Unable to verify my payment information for buying iCloud plan

This is a user-to-user forum. Apple largely is not here, except to moderate it.

You need to contact Apple Support about your payment method issue.

If your payment method is declined in the App Store or iTunes Store - Apple Support

Official Apple Support

Note that you cannot purchase additional memory (RAM) or device storage for an iPhone. Whatever it comes with from the factory is what it will have forever.

You can rent iCloud+ storage, for backups and synchronization.

What's the difference between device storage and iCloud storage? - Apple Support