Storage
I have 256GB storage
why can’t I back up my phone
5GB cloud storage is full
iPhone 11, iOS 14
Question marked as
The 256GB is the storage on your iPhone. The backup needs to be stored someplace other than your iPhone -- a major purpose of a backup is to be available if the phone is damaged or lost.
If you don't want to pay for more iCloud storage, another option is to backup to a computer. See: How to back up your iPhone, iPad, and iPod touch - Apple Support