Store all files, photos and messages in iCloud and save space on your Mac. You’ll see one or more of the following options, based on how you’ve set up storage management in iCloud, Photos and Messages.
- Desktop and Documents: Store all the files from your Desktop and Documents folders in iCloud Drive and optimise storage by keeping only recently opened files on your Mac when storage space is needed.
- Photos: Store your photo library in iCloud and optimise storage by keeping only optimised photos and videos on your Mac when storage space is needed.
- Messages: Store all messages and attachments in iCloud and keep only recently opened items on your Mac when storage space is needed.
You can turn off storage management by modifying the related settings in iCloud preferences, Photos preferences and Messages preferences. See Store your desktop and documents in iCloud Drive, Use iCloud Photos to store, share and manage your photos in iCloud, and Use Messages in iCloud.
You have already set up iCloud photo library https://support.apple.com/en-in/HT204264 and photo box is turned as on in the location system preferences > iCloud so you don't need to configure two times , the error will show .
if you click on store in iCloud .