Unwanted Security Alert

If it's while surfing, it's a scam. No web site can check your system for viruses. It's just a trick to try and get you to contact some scammer's fake "support". Try going to Safari's Preferences, click the Privacy tab, then Manage Website Data and remove everything. You'll lose cookies and cache, but that may remove whatever might have gotten into your cache that's causing this to keep popping up.

If the problem persists, download Malwarebytes:

https://www.malwarebytes.com/mac

and have the free version check your system.

Regards.

If this is opening the last page that you viewed when you launch Safari, then quit Safari and hold the Shift key and launch it again. This will give Safari a fresh start. It is common to see these alerts when viewing websites that are less then legitimate.

If you see Notification in the upper right of screen, then go to Safari > Settings > Websites > Notifications, and remove all websites you see here.

Stop unwanted Notifications - Apple Community is applicable to that particular problem.

Those Notifications are completely fraudulent and they not arise from Apple. Do not install anything in an effort to get rid of them. Stop unwanted Notifications - Apple Community describes how they arise and how to stop them.

FTC - How to Spot, Avoid, and Report Tech Support Scams