The message clearly says that a required Windows Service could not be started. Here are the areas you need to take a look at to fix this Microsoft Defender error: Check Windows Defender service status ...

Toggle the Real-time protection switch off. You may receive a User Account Control (UAC) prompt asking if you want to allow the change to occur. Select Yes. When you're ready to turn system protection ...

The first thing you should do is click on the Restore Settings placed right below the error message and see if the issue is resolved. Sometimes, Windows revert back ...

Windows Defender is a capable antivirus solution built into Windows itself. Unless you’ve installed a different antivirus program on your Windows 11 or Windows 10 PC, your PC is using it right now.