" a required download is missing"

[Sully_94]

I have tried to factory reset my 2011 iMac after backing it up by pressing "cmd-option-r". I have gone to erase the disk in disk utility and when I go onto reinstall OS X Mavericks is says a required download is missing. Some people are saying to change the date, which I've tried doing and the same error message pops up. I've also seen people are saying use a bootable USB to install, not sure what that is. If someone knows how to help it would be much appreciated

 

[Fishrrman]

I'd suggest you try OS 10.12 (Low Sierra) instead of Mavericks.
I think you can find it here:
macOS Sierra 10.12
Additional info from the source:
"Your web browser downloads the following older installers as a disk image named InstallOS.dmg or InstallMacOSX.dmg. Open the disk image, then open the .pkg installer inside the disk image. It installs an app named Install [Version Name]. Open that app from your Applications folder to begin installing the operating system."