OS X could not be installed on your computer error

Having an awful times trying to reset my old iMac for my mum. Went through all the instructions to reset the settings and delete my profile from it, and yet after every restart it was still all my stuff, my Apple id etc.

Apple instructions said to erase the disk and reinstall the operating system. Erased the disk in disk utility, then went to reinstall OS X. After nearly 3 hours screen changed to say “OS X could not be installed on your computer. No packages were elligible for install. Contact the software manufacturer for assistance. Quit the installer and restart your computer and try again”. Tried restarting, same result. Why on earth is it so hard to reset to give to someone else? How on earth can I fix this? Any help please would be so greatly appreciated, so far this has taken 2 full days and lots of stress.