How do I get rid of a notification that I have a virus

Go to Safari > Settings > Websites > Notifications. Remove any website you allow Notifications for. If you are looking for the particular site causing the problem, you may be able to match the icon displayed in the Notification with one listed on that Settings page.

That should solve your problem and good for you for not falling for this scam by believing you have a virus. This notification was inadvertenly allowed from a website that may have tricked you into believing that the notification would provide useful information. In most cases it is best to never allow a website to provide notifications.