I have a pop up notice with the apple icon that says: "The System is in danger. Click here to turn on Antivirus"

Learn how to block annoying pop-up ads in Safari on your iPhone, iPad, iPod touch, or Mac using the tips in this support article —> Block pop-up ads and windows in Safari - Apple Support

It is a bogus warning trying to get you to buy something you don't need. At worst it is phishing.

Scams (e-mail, text messages, and phone calls) are getting very good at closely imitating true Apple communications. Always be cautious. These support articles have some guidelines:

About identifying legitimate emails from the App Store and iTunes Store --> Identify legitimate emails from the App Store or iTunes Store - Apple Support

Recognize and avoid phishing messages, phony support calls, and other scams - Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Avoid scams when you use Apple Cash --> Avoid scams when you use Apple Cash - Apple Support

Apple 'How to identify, avoid, and report phishing' video--> https://youtu.be/SR3Z3fXXjfw

If you are uncertain about a message and a resource provided in that message, do not click on any links in the message. Try to independently verify the resource by going to a support page on apple.com and use that to contact Apple or to verify with the service the message is telling you to use.

Are those messages appearing in the upper right of your display? Those are Safari Notifications and they're a scam. Just another variant of a phishing attempt.

Fix it. Go to Safari's Settings (or Preferences) menu > Websites > Notifications, and remove what you do not want — which is probably everything.

For reference: Change Websites settings in Safari on Mac - Apple Support

Don't allow websites to send you Notifications unless you are 100% certain that you want that kind of harassment. Refer to Customize website notifications in Safari on Mac - Apple Support. Deselecting "Allow websites to ask for permission to send notifications" will prevent those websites from even asking your permission to be harassed by them.

No, all such messages are scams presented by criminals. Are you seeing this as a message when browsing? Or, as a “System Notification”?