Computerworld - https://www.computerworld.com/article/3255625/apple-mac/mac-what-does-system-scan-is-recommended-mean.html#tk.rss_all
Many Mac users may have come across a small window that appears on top of their browser when surfing the Web that warns them, ‘System Scan is Recommended’. So, what is this message, and what should you do if you see it?
TL;DR: Don’t panic
The first thing to learn is that this is not a Mac system message. If you ever come across this message you can be utterly certain that it is a scam. Whoever is behind the message (and it may not be the website owner, but some poorly policed ads network) wants you to agree to something