My iMac cannot see my printer at all

Try resetting the printing system and adding the printer again If you can't print from your Mac or iOS device - Apple Support

If both devices were running 10.15.4 and the MacBook was able to see and use the printer then that rules out the system as the blockage.

Did you run any installers/updaters, cleaning or optimizing apps between the time you could use the printer and when you couldn't?

Hold down the Option key and in the  menu select System Information. In the SI window select Installations in the sidebar. You can sort the installations by Install Date to find those around the date you're looking for.

Do your iMac and MacBook run the same system?

Ask yourself if there are no mac viruses why am I installing AV software? As you've discovered AV software on a Mac is less than useless and only serves to cause problems. Uninstall Macafee per the developers instructions.

Don't contact MacAfee except to get a refund. It's a total worthless app for Macs. There are no known viruses for Macs and MacAfee is not designed to effectively search for adware or malware. It just sits there and uses up system resources. Go a Google search for how to uninstall the MacAfee software.

Also If you're behind a router you don't need to activate the Firewall. The router does an excellent job of preventing access to your Mac.