How do I stop repeated alert saying"'Mac OS: The system is in danger!" on my Mac?

It's a scam message pushed to your computer by a Web site. The threat is from the criminals who sent it.

The User Tip that John Galt mentioned will show you how to block messages like these from coming through Safari. If you use other browsers (Firefox, Chrome, etc.), also make analogous changes to their settings.

Please read Stop unwanted Notifications - Apple Community.

Go to Safari > Settings > Websites > Notifications, and remove all websites you see here. You have allowed a website to send you these notifications. Your computer is not in danger.