It is not a virus and there is nothing wrong with your computer. What you did was allow a website to send you Notifications, and it is taking that opportunity to scare you by believing you have a problem and will download some software to fix a problem you do not have. Think of it as a way to send you an advertisement, and it is done by less than legitimate websites.
Go to Safari > Settings > Websites > Notifications, and remove all sites you have allowed to send you Notifications. Your problem will be solved.
If you did download some antivirus software, then remove it now. If you need help with that, than you would need to provide more details on what you did.