Back up failed
My iPhone 12 is not backing up. I get a message that my iCloud is full, but when I check my iCloud, it is far from being full.
Which iPhone 12 do you have? , with 64, 128 or 256 GB?
Then check which iCloud subscription do you have, go to -> Settings -> your name -> iCloud -> if you see 50 GB (or 200 GB or 2 TB) then you have an iCloud subscription.
If you have an iPhone 12 with 256 GB, you will need 2 TB iCloud subscription.
See also -> Manage your iCloud storage -> Manage your iCloud storage - Apple Support