Windows requires that the user have the correct privilege (SE_SHUTDOWN_NAME) to be able to shutdown or reboot the computer and that the process requesting it (i.e. SyncBack) is run elevated.

As an administrator you have that privilege, but not as a standard user. As an administrator you can give this privilege to a user:

  • Start the local security policy (secpol.msc) manager (in Windows 8 (or newer) Pro or Enterprise version only)
  • Go to Local Policies -> User Rights Assignment
  • Double-click on "Shutdown the system" and add the user