MacOS High Sierra Installation Failure

I have a MacMini (mid 2010) that currently has MacOS High Sierra installed. However, I need to do a "clean" re-install of High Sierra, but have not been able to complete the install process. I have used macOS Recovery process and have also tried using a separate bootable recovery USB dongle.

The process proceeds for a few minutes and then I get an error message: "An error occurred while preparing the installation. Try running again." Trying again has the same issue.

Next I went to Disk Utility and ran Disk First Aid. I then get the message: "First Aid could not unmount the volume for repair."

Do you have any suggestions on how I can resolve this issue so that I can do a clean install of MacOS?

Thank you.