I keep getting a pop up that says to install anti-virus software

If you are using Safari, check the settings at: Safari > Settings > Websites > Notification. It is best to block all Notifications and then confirm the problem is gone. After that only allow Notifications for sites that are known to be reputable and that you want to receive Notifications from them.

The same is true for the setting at: Safari > Settings > Websites > Pop Up Windows.

Another thing to consider is never install Anti-Virus or Cleaner apps. These will only cause problems with your system and some of them can be considered Malware with constant prompts saying you need to install additional software.

It's a scam; ignore it. There's nothing to clean.