I'm currently running Sierra, trying to download the El Capitan installer from the App Store, but am unable to due to the message in the title. Is there any way around this, or am I forced to erase my HDD and download an OS from internet recovery mode? Am I able to pick a specific OS to download from recovery mode if that was the case? To note: I don't have a back up system to restore to. I have a drive to load the installer on to. Thank you.