Are you seeing it in the upper right corner of your Mac as an actual Notification?
If so, it is a fake notification brought to you by a website that you have allowed to send you those notifications. To get rid of it, open Safari, then go to Safari > Settings > Websites > Notifications, and remove all websites you see there.
If that is not it, Websites can pop up anything they want and are paid for ads that claim such things so you will download some kind of software. In most cases you can close the page or navigate to a different one. If you find you are unable to do that, quit Safari, then hold the Shift key and open Safari again. This will open Safari to the default page without showing the last page opened.