Microsoft is investigating a Windows 7 bug which has confronted users with errors when trying to shutdown their machines. Although the root cause of the problem is still unclear, a solution has been found by users on Reddit and other community sites.
This guide applies to Windows 7 machines which started displaying a "You don't have permission to shutdown this computer" warning this month. This particular method requires access to the Group Policy Editor, which is only available in Windows 7 Professional and up.
Launch Group Policy Editor by pressing Win+R and typing "gpedit.msc" into the Run box. Next, use the tree view to the left of the Group Policy Editor window to navigate to Computer Settings > Windows Settings > Security Settings > Local Policies > Security Options.
Find the "User Account Control: Run all administrators in Admin Approval Mode" policy. Double-click it and change the policy's status to "Enabled", if it isn't already.
Next, launch Command Prompt (press Win+R again and type "cmd"). Using the Command Prompt window, type "gpupdate /force" and press enter. Next, type "shutdown -r" and press enter to force a reboot of your machine. When it starts back up, shutdown should work normally.
This technique works by forcing administrator user accounts back into Admin Approval Mode. Admin Approval Mode causes administrator accounts to prompt for permission when an app requests elevated privileges. By reapplying the policy, the shutdown permissions error can be remediated.
Although Windows 7 is now unsupported, Microsoft confirmed to Bleeping Computer that it is "actively investigating" this issue. It comes after Microsoft released an unscheduled update for Windows 7 to fix a desktop wallpaper bug. This problem seems to be much more serious, so it may be that Windows 7 receives one more patch yet.