You are getting the error code - 50021 when you try to install the following Windows updates on your endpoints.
The problem is because of the following reason.
Microsoft has acknowledged this in an official statement that "Microsoft and Symantec have identified an issue that occurs when a device is running any Symantec or Norton antivirus program and installs updates for Windows that are signed with SHA-2 certificates only. The Windows updates are blocked or deleted by the antivirus program during installation, which may then cause Windows to stop working or fail to start."
To resolve this issue, upgrade your Symantec Anti-virus software to the latest version and then install the above mentioned Windows updates.
For more clarity, Refer the official KB document from Symantec.
If problem persists, contact support