2013 iMac won’t reinstall is after factory reset

Hi,

i’m all but computer illiterate but try to factory reset my 2013 iMac as it’s been in a box for years and I thought it might work quicker with a clean up.

I’ve somehow manage to uninstall all the software and when I’ve tried to reinstall OS (it’s only giving me El Capitan option), it spends an hour loading it then I get a message saying ‘OS X could not be installed … no packages were eligible for install’

is there anyway for me to reinstall some form of OS and is anyone able to give me an idiots guide, and I can’t stress the idiot part enough.