How to backup from iCloud to my iPhone

Are you trying to restore an iCloud backup? Restoring from a backup erases all data currently on your device. Any data newer than when the backup was made will be erased. A restore is an all or nothing restore. You can't access a backup to see the contents and only restore certain files.

Backup - Restore your device from an iCloud or iTunes backup.    *

Or are you trying to backup to iCloud?

Backups – Choose Method.  -

Back up your iPhone, iPad, and iPod touch.  *

Back up – What does iCloud back up.  *

https://www.youtube.com/watch?v=EvEqiO7rWlA&list=PLIl2EzNYri0fwwkKJtNNX2CksbIqfUuql&index=2

See if anything here helps

Backup couldn’t be completed.   -

Backup couldn’t be completed or you can’t restore.  *

Backup couldn’t be completed or you can’t restore (2).  *

Backup couldn’t be completed or you can’t restore (3).  *