Chosen Solution
You’re hitting a known issue! You need to move up to a newer macOS version as OS X Mountain Lion certificates have long since expired! Here’s a more detailed write up If you’ve got an old macOS install image, it will probably stop working today There is a way to get around this but you need to work off a local OS installer from a USB drive. Basically we will need to alter the systems date to a date thats within the certificates lifespan.
But you need the Mountain Lion OS installer setup on a bootable drive. I think its a lot easier getting a newer OS installer either from internet recovery using the Option - ⌘ - R keys. Reference: How to reinstall macOS from macOS Recovery Or, setup a bootable OS installer following this guide How to create a bootable macOS Catalina installer drive Using the Apple Store How to upgrade to macOS Catalina image file.
