( You may see 'Windows Defender Antivirus' instead, this is the same thing.)

If you see 'Microsoft Defender Antivirus is turned on' you are using Microsoft Defender as the primary AV.

You will not be able to activate Microsoft Defender until the third party product is removed from your system. You should not have a lapse in protection, as Defender will take over when the third party product is unregistered.

Uninstall the third party AV so Microsoft Defender antivirus can be turned on. If you have a third party AV listed (like Symantec Endpoint Protection) you may see that it is turned on and that Microsoft Defender antivirus is turned off.

Check which product is listed as 'turned on' under antivirus.

From the options on the right, select manage providers under 'Who's protecting me?'.

Confirm Microsoft Defender is the Primary AV Windows 10 and Windows Server 2019 These settings should be available through the 'Update & Security' section in the main Settings app, searching for Windows Security or Windows Defender in the start menu, or by searching directly for virus & threat protection. Note: Different versions of Windows 10 may have slightly different steps to access the Microsoft Defender settings. You must uninstall any third party AV before Microsoft Defender can be used as the default AV again. If you installed a third party AV, Windows Defender cannot be re-activated as the default AV for your system. Microsoft Defender will continue to provide passive scanning capabilities even if a third-party AV is installed. (Some systems may refer to this still as 'Windows Defender.') Microsoft Defender comes pre-configured on Windows 10 and Windows Server 2016 to provide standard antivirus (AV) scanning and protection by default. Enabling Microsoft Defender on Windows 10 and Windows Server 2016 and later

How to enable microsoft defender