How do I know my MacBook Pro has Virus?

If the messages appear in the upper right-hand corner of the screen, they are probably Web notifications. You can block those for Safari by following the instructions in this User Tip, and block them for any other browsers you use by setting their versions of the same preferences.

Stop unwanted Notifications - Apple Community

If the messages pop up when you visit a Web site, close out the Window (force-quitting the browser if necessary), and avoid going back to whatever Web site presented the scam to you.

FTC – How to Spot, Avoid, and Report Tech Support Scams

What says you have a virus? Unless you have an anti-virus application installed on your Mac, and ran a virus scan, this is very likely a scam message from criminals.

Of course it does. It is an advertisement trying to get you to purchase their software or call a number and give them your personal/financial information. If they told you everything was fine, they wouldn't be able to scam you.

No one, not even Apple, can "see" what is on your computer unless you give them express permission to remotely access your computer.

One of the oldest scams in the book.