Need to purchase more storage

You can subscribe to more iCloud storage but you cannot increase the amount of storage on the phone itself. See the information in What's the difference between device storage and iCloud storage? - Apple Support and Upgrade your iCloud storage plan - Apple Support

Sorry, you can not add storage space to an iPhone. If you need more space, you will need to delete or move existing content.

There is no way to increase an iPhone’s storage. You must purchase a new iPhone to obtain additional storage.