Event ID - 21

Event Id21
SourceMicrosoft-Windows-WindowsUpdateClient
DescriptionReboot required: To complete the installation of the following updates, the computer must be restarted. Until this computer has been restarted, Windows cannot search for or download new updates: %1
Event InformationAccording to Microsoft :
Cause
This event is logged when to complete the installation of the following updates, the computer must be restarted.
Resolution
Restart the system
If updates are available but are not automatically downloaded, restart the system.
Verify
To confirm Windows Update Agent has installed updates:
1.Open an elevated Command Prompt window. Click Start, point to All Programs, click Accessories, right-click Command Prompt, and then click Run as administrator.
2.At the command prompt, type eventvwr.msc and press ENTER to open Event Viewer.
3.To check for events in Event Viewer:
a.In the left panel of Event Viewer, click Application and Service Logs.
b.Expand Microsoft, and then expand Windows.
c.Click WindowsUpdateClient, and then click Operational.
d.Check to see if Event ID 19 is present in the event list to confirm that Windows Update Agent has successfully downloaded the updates.
Reference LinksEvent ID 21 from Microsoft-Windows-WindowsUpdateClient

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