Pop up saying I have a virus

You don't have a virus...maybe Malware...download the free version here...https://www.malwarebytes.com...then run it...if there is something on your Mac it will remove it.

The next time you get such a popup just close the browser...if it persists hold down the power button until your Mac shuts down.

Don't go back to a site where the popups come from...choose another site...it's coming from the sites you visit.

As fyip alludes to it's a scam, which Browser are you using?