i can not purchase more storage for my iphone

Please note that, while you can purchase more iCloud storage for backup and syncing, you cannot purchase more local storage for your phone.

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

See -> Related Article -> Upgrade to iCloud+

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Upgrade to iCloud+ from any device

Whether you’re on an iPhone, iPad, iPod touch, Mac, or PC, upgrading to iCloud+ is simple. Choose your device below and buy more in just a few quick steps.

How to upgrade on your iPhone, iPad, or iPod touch

1. Go to Settings > [your name] > iCloud > Manage Storage or iCloud Storage.
2. Tap Buy More Storage or Change Storage Plan.
3. Choose a plan and follow the onscreen instructions.

If you're on your iPhone, iPad, or iPod touch with iOS 14 or iPadOS 14 or later, tap here to upgrade to iCloud+.