OS X EL Capitan..
Why can't I install OS X El Capitan in old iMAC? It would only say "this item is temporarily unavailable" after I log in my account. Pls help.
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Recently certificates for older install apps for certain macOS's expired meaning they could no longer be installed.
If you are using an old bootable USB installer you created a while ago then that is the error message you will see.
One thing you can do as a workaround is to change the Date & Time of the mac to a date in 2018 for example.
The installer should now work, when the installation has finished you can change the Date & Time back to today.
Apple did release updated installer files at the end of October, you can get El Capitan here,
How to upgrade to OS X El Capitan – Apple Support
At Section 4 download InstallMacOSX.dmg. When that has downloaded open it and you will get InstallMacOSX.pkg.
Double-click on that and an installation window will open, this will convert InstallMacOSX.pkg to the
Install OS X El Capitan.app which will be in your Application folder.
Use the Install OS X El Capitan.app to create a new bootable USB installer, How to create a bootable installer for macOS – Apple Support
Please note you can only convert the InstallMacOSX.pkg to the Install OS X El Capitan.app on a mac that can install El Capitan.
Therefore if your mac came with macOS Sierra, macOS High Sierra, macOS Mojave or macOS Catalina pre installed you will not be able to convert the InstallMacOSX.pkg to the Install OS X El Capitan.app.