My IMac is saying required download is missing.
My father recently purchased an IMac from someone and we tried to erase it to factory settings so after erasing the drive we go to reinstall the OS and it says a required download is missing and I tried to use the first aid to fix it and see if it would help but it does nothing. I'm not sure what to do and I'm lost because I don't know where to go from this point. If anyone has any idea please let me know, I was thinking of downloading it onto a usb and trying it that way but I think it said I need a mac to do that. I am desperate for help and out of ideas. My version is el capitan. thanks!