Keep getting notices that I have viruses

They are all bogus and scams. There is no one in the world who would know what is on your computer. They are simply wanting your private information and your money.

And, since there are no viruses for a Mac, there is absolutely no need to install AV software as it will usually either slow down your system or cause havoc with it.

The messages are likely bogus (unless they are coming from an anti-virus checker you have installed on your system). A computer without anti-virus software on it, for the most part, cannot alert you that you have a virus.

Could be a byproduct of a website you are currently on or have visited in the past. These types of messages want you to click on them so they can do bad stuff to your computer. Most of the time you should close the window that has the message in it immediately without interacting with it.