I tested this myself on a 2012 Mac mini running Catalina. What you'll need to do is change your system date for it to work. Otherwise it says the El Capitan installer is too old. Click the link below for directions. You need to boot into recovery first. Then open Utilities/Terminal and enter the date.
i tried changing the system date but no luck :( im starting to lose hope about getting this to work You can download El Capitan here . When you come to the installation screen go to the utility then terminal . In terminal type this " date 000001012016 " to change the system date . Quit terminal...
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