Hello,
The given icon indicates either a connection or storage issue. First check if you have enough available space for the files you'd like to back up. You can check that in System Settings >> Your Name >> iCloud to see how much iCloud storage you have left. You can manage your iCloud storage here: Manage your iCloud storage - Apple Support
If not the storage, then there may be a connection issue with iCloud. In System Settings >> Your Name >> Sign Out of your Apple ID at the very bottom left and then sign back in and check to see if there is still a notification on the iCloud icon. Hope this helps.
Yasamean.