Symptoms
Plesk Installer shows the following warning:
System checking...
The following services require a system restart:
Component-Based Servicing pending restart
Windows Update requires restart
Restart the system before continuing installation.
Cause
The system has updates that require a reboot.
Resolution
Restart the server using one of the following methods:
via Plesk GUI
Go to Tools & Settings and click Restart Server (under Server Management).

Once the server is started, rerun Plesk Installer.
via RDP
Connect to the server via RDP.
Click on Start > Restart to restart the server.
Once the server is started, rerun Plesk Installer.