If your Defender client is severely out of date (months behind), the delta package (the small update) fails. You need the full platform update.

KB915597 - Security Intelligence for Window Defender Antivirus

(formerly known as definition updates) for Microsoft Defender Antivirus and other Microsoft antimalware solutions

Get-MpComputerStatus | select AntivirusSignatureVersion

: For users who need to manually download and install definition updates, the Microsoft Update Catalog website provides a searchable database of updates, including definition updates for Windows Defender.

Important System Requirements

Download 2021 Definition Update For Windows Defender Kb915597 Definition 155 11190 Better -

If your Defender client is severely out of date (months behind), the delta package (the small update) fails. You need the full platform update.

KB915597 - Security Intelligence for Window Defender Antivirus If your Defender client is severely out of

(formerly known as definition updates) for Microsoft Defender Antivirus and other Microsoft antimalware solutions including definition updates for Windows Defender.

Get-MpComputerStatus | select AntivirusSignatureVersion If your Defender client is severely out of