If you see such an error, it means that the backup file on your child's iPhone/iPad is damaged.
Please follow the next steps in order to fix it.
1) Delete the backup
Go to Settings -> iCloud -> Storage, then tap Manage Storage.
Tap the name of a device, then tap Delete Backup. After you do this, create a new backup and check the mSpy account after that.
2) Do a manual backup
Go to Settings -> iCloud -> Back up - hit the BACK UP NOW button. And wait a bit till it gives you the estimated time of back up.
3) Wait for 24 hours and then link your device
Login to your mSpy account here, go to the 'Device management' tab at the left-hand side list and go to 'Step 3' -> input iCloud credentials -> hit 'Verify'
Select the backup with the recent date and then 'Proceed'.