Check the Microsoft Defender icon near the clock on the bottom right corner of the screen. (You may have to click on the up arrow to see it).

If the icon shows an exclamation mark , please open it and attend to the cause creating it.

For example, in this case, the exclamation mark is being caused because Kernel-mode Hardware-enforced Stack protection is OFF. Put it ON

If asked to restart the system, please restart.

Verify that the icon is now with a green tick .