I was trying to reinstall an old Apple MacBook Pro from 2009 for a friend. They had previously upgraded all the way to the newest operating system macOS Sierra version 10.12.6 and they wanted to do a clean installation to sell the computer on eBay.
He booted in recover mode Command-R at startup, did the usual, first formatted the drive, then went to do the installation. Should be very easy.
Recover mode, gives the option to reinstall OS X El Capitan. I know this machine was working fine, but I’m getting an error when trying to run the installer.
Error message: An error occurred while preparing the installation. Try running this application again.
I remember having this problem once before. After wasting an hour messing around, I noticed the clock date was reset to 1/1/2001. They must have left the computer unplugged and the battery died and reset the system time.
I then opened up Terminal while in the recovery menu and reset the date to today. Just type in “date” with format mmddHHMMYYYY, example:
date 020416002018
mm = month
dd = day
HH = hour
MM = minute
YYYY = year
That will set the computer to 2/4/2018 at 4:00pm, enter your current date and time.
Now quit Terminal application, and run the OS X El Capitan installer again.
Everything should be good now.