iCloud

You may to either clean out iCloud - Manage your iCloud storage - Apple Support, or you may not have enough on-device storage space - How to check the storage on your iPhone, iPad, and iPod touch - Apple Support.

About storage on your device and in iCloud - Apple Support

Manage your photo and video storage - Apple Support

You cannot as that's not how iCloud works or was designed. iCloud serves as a syncing service that syncs all of your photos/videos across all devices. What you may be looking for as it relates to what you're asking is something like Dropbox or Google Photos.