Even if you did a clean install of El Capitan it would not explain why you are seeing OS X 10.6.8. OS X 10.6.3/4 was the original system on your computer. Still, it’s hard to know where the 10.6.8 came from, unless Time Machine has been disabled or disconnected for the last 8 years or so. What is the date of your latest Time Machine backup?
Dec 01, 2019 How to create a bootable installer for macOS - Apple Support Mar 12, 2020 How To Reinstall Mac OS on Your Mac - macReports Oct 09, 2016 How to Reinstall OS X Lion: 4 Steps (with Pictures) - wikiHow Mar 29, 2019
'An Error Occurred While Preparing the Installation' Fix
locked hard drive can't reinstall Lion OSX. @danj also suggested in this solution to create a separate bootable thumb drive to reinstall OS X from, erase the disk …
Dec 01, 2019
SOLVED: Can’t reinstall MacOs because disk is locked (2nd