Advise for browser viruses

Apple won't send you a message like that. You may  have adware/malware installed. Try downloading this program which was written by Thomas R, a long time poster. The program will search for malware/adware. The program will do the work for you which makes it easy. It may be necessary to run more than one scan. After running the scans, restart the computer.

Malwarebytes Anti-Malware for Mac      10.10 and later

Malwarebytes uninstall

As Eric has stated that message is not from Apple. For starters there is no way for them to know what is installed and running on your Mac. That right there should tell you they are lying to you.

Now that you know they are liars, are you still going to trust them?

My suggestion is to follow what Eric has told you.