Apple gives the option to password protect device backups, so this password that is being requested is a password you (or your child) created when you were doing a backup to the device through iTunes, in the past.
Please enter the unique encryption passcode created when encryption was first enabled in iTunes for this device.
If you or your child can't remember the password that was created to password protect the backup, it will not be possible to move forward with Premium iOS installation at this time; unfortunately, Apple does not provide a password retrieval or reset mechanism. If this is your case, we encourage you to downgrade to our PLUS tier of management, which does not require a device backup to be completed.
You can learn more about Apple-encrypted backups here, or by contacting Apple Support.
Please email us at firstname.lastname@example.org if you have any questions!