Right-click on the Start Menu button to open the Win X Menu.In the Win X Menu, click on Command Prompt (Admin) to open an elevated Command Prompt.This error can occur when the client encounters an error while trying to load the definitions, or if the file is corrupt.
Type exe -signatureupdate into the Command Prompt and press the Enter key.
Once the updates are downloaded, they will be installed – successfully, this time.
You can also try updating Windows Defender using an elevated Command Prompt.
This event is usually due to an interruption in network connectivity during an update.
To troubleshoot this event: The Windows Defender client attempted to download and install the latest definitions file and failed.