You can see if anything in here helps, but it really just contains some basic suggestions that you may have already tried.
If you can't back up to iCloud or restore from iCloud backup failed - Apple Support
I would suggest force restarting the phone and then try again. A forced restart cures so many issues for me. Not sure which phone you currently have so here all the various ways to force restart.
- iPhone 8 or later: Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then press and hold the Side button until you see the Apple logo.
- iPhone 7, iPhone 7 Plus, and iPod touch (7th generation): Press and hold both the Top (or Side) button and the Volume Down buttons until you see the Apple logo.
- iPad with Home button, iPhone 6s or earlier, and iPod touch (6th generation) or earlier: Press and hold both the Home and the Top (or Side) buttons until you see the Apple logo.