How to buy more space on my iPhone?

Note there's different kinds of "storage". Click on this link to learn the difference between your device and iCloud storage, ways to manage your space, and upgrading to iCloud+ if you need more --> What's the difference between device storage and iCloud storage? - Apple Support

Also try some of the things suggested in the message you are showing us.

If you need more physical storage for apps, then it’s worth looking into upgrading your iPhone. 📲

If you need more file storage for photos, videos, documents and messages then consider adding ➢ iCloud+ for details. ☁️

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• Chat now or call 1-800-APLCARE.
• International Support.