System is under threat pop up

Past couple days I'm getting Pop up windows on our new MacAir warning that the system is under threat. Most recent was while reading a news article and the pop up window appeared; the page was black with a large white pop up covering another rectangle shaped pop up window - the pop up message: "MacOS Security Center has identified that your system is under threat. Please san your MacOS as soon as possible to avoid more damage. Don't leave this page until you have undertaken all the suggested steps by authorised Antivirus." (I tried to insert a photo here - but kept getting an error message).

I closed out Safari and re-booted the computer (as I'd been told to do when this happened a month or so ago. I checked for unfamiliar profiles and apps and there are not any. What else can I do - this has happened two days in a row now -- Do I need to do a re-install?

Thanks for any suggestions.