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I erased my hard drive once before selling it but to be safe, I decided to do a secure erase and now the person is not able to install the software to get it up and running. Help please

When you erased the drive you removed the OS files as well. You’ll need to aim your friend to this Apple TN: About macOS Recovery or give them a USB thumb drive configured as a OS installer following this guide: How to create a bootable macOS High Sierra installer drive using your new system