Cannot reinstall MacOS I have erased my Mac Mini and now I am presented with the Recovery screen where I can use Disk Utility, Reinstall, etc.

Nothing works!

Disk Utility claims it cannot unmount the primary HDD, despite the "mount" option being greyed out, which suggests it's not mounted. Disk Info says "Writeable: No".

When I attempt to reinstall, I get to the HDD selection screen but the only option is to select the Recovery HDD, which is also greyed out LOL!

The Mac Mini came with no CD (ordered from Apple). Everything was preloaded. Apple website suggests downloading the MacOS to create a bootable USB... But you need a Mac to do so!!!

Don't know what else to do and would be really grateful if anyone could share what I can do.
You need to create a new partition using Disk Utility that you can use for installing macOS. 

You'll need somewhere to store your system, and without a partition (besides the recovery partition), you'll have no such place. That's why you have no choices when you run the installer.