I suggest you to disable and enable the Windows Defender feature and check if the issue persists. Follow the steps below to disable Windows defender.
1 Press Windows logo key on the keyboard, type Windows Defender Settings and chose the top most search result.
2 Switch the slide bar under Real-time protection to Off. Wait for few minutes.
3 Slide the bar to On and check if the issue is resolved.
Disclaimer: Antivirus software can help protect your computer against viruses and other security threats. In most cases, you shouldn't disable your antivirus software. If you have to temporarily disable it to install other software, you should re-enable it as soon as you're done. If you're connected to the Internet or a network while your antivirus software is disabled, your computer is vulnerable to attacks.
If the issue persists, check if you have enabled the Windows Defender Service by following the steps below:
1 Press Windows logo key on the keyboard, type Services and select the top most search result.
2 From the window locate Windows Defender Service and check if Status shows Running and the Startup Type is set to Automatic.
3 If not set, double click on the service and select from the drop down bar Automatic under Startup type and Service status to Running.
4 Click on Apply and OK.
5 Close the window and check if the changes are effective.