Restoring system partition requires the system to reboot before restoration starts
Windows does not allow you to run an application while this application completely rewrites the partition where Windows is installed. Therefore, Acronis True Image will prompt you to reboot the computer into its standalone version to restore the system partition. Once the partition has been restored, you will be able to boot Windows in the usual way.
For additional information on the standalone version see Standalone Version of Acronis Software.