problem reinstalling OS X El Capitan
trying to reinstall El Capitan on a MacBook Pro. getting the message 'A required download is missing.' What do I do?
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Is the copy of El Capitan you are retrying to install an old copy you downloaded some time ago.
Recently the certificates for certain Mac OS's expired making it difficult or impossible to install them.
Apple released updated versions with new valid certificates, so you could try downloading the updated
release of El Capitan, from here. How to upgrade to OS X El Capitan – Apple Support
Please note there are different instructions, read Sections 4 to 7 completely.
At Section 4 you click on Download OS X El Capitan, this will download Install MACOSX.dmg.
Double-click on Install MACOSX.dmg and you will then get Install MACOSX.pkg.
Double click on that and an installation window will open, this does not install El Capitan but converts
Install MACOSX.pkg to Install OS X El Capitan.app which will be in your Application folder.
Use that to install El Capitan.