Why I am getting error 2100F, During O/S setup on Mac Mini A1993 ???

Hello there mrt21,

Thanks for posting here on the Apple Support Communities! This is a place where other Apple users can submit questions or respond to others to help lend a hand. Let's take a closer look as we do our best to provide some additional insight.

The error you're seeing there usually indicates an issue with your network, or something is blocking certain ports required for a secure connection to Apple's servers. First, be sure to follow the steps here under the "If you can't start up from macOS Recovery" section: About macOS Recovery on Intel-based Mac computers

Also, make sure your router and network are using Apple's recommended settings: Recommended settings for Wi-Fi routers and access points

We hope this helps get you pointed in the right direction.

Take care.