How do I add iCloud storage on my iPhone
Why can’t I add storage to my iCloud account
Question marked as
Why not? What happens when you try? Are you getting an error? If so, what does it say?
Are you following the steps here? : link ➡️ Upgrade to iCloud+ - Apple Support
Do note, iCloud storage in on an Apple Server, it does not affect your iPhone's local storage capacity in any way.