That's spam. Ignore it. If that website could do what it claims, it wouldn't bother running the fake "scan" and would just rip off you and your data directly—websites cannot rummage the file system as anti-malware scans would involve, and even local apps can't do that.
There's a ~50-page thread around of folks asking this same question.
I got a popup that says “Your system is infected… - Apple Community
More than a few of the scams target the user and fooling the user, and particularly encouraging the user to do something contrary to the user's interests. "VIRUS" is great for scaring users, too. Here's the Apple article on this:
Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support
The local rule of thumb, around websites and popups and advertisements:
if you didn't go looking for it, you probably don't want it.