system mac os is infected popup

Hi Jennifer, I hope you managed to get rid of this by now but in case you haven't: The notification is caused by either a browser extension or website. I recommend running Malwarebytes Anti-Malware to remove this. You should be able to just use the Home version, so no need to pay. Just make sure to avoid the upsell during installation.

I recommended the same to a colleague of mine and it removed a Chrome extensions which was triggering those notifications.

I would be very vigilant when websites ask to allow notifications or installing browser extensions from non-reputable publishers. It's often the case they are malware/adware themselves or have weak security which allows bad-actors to hijack their service to push fake notifications.