el capitan not available

El Capitan remains available for download, from here,

How to upgrade to OS X El Capitan - Apple Support

As you have not provided any information on the model and age of your MacBook or what OS it is running it is difficult to give you exact advice.

If your Mac can upgrade further than El Capitan you will be refused the download, this means you can possibly upgrade to either Sierra or High Sierra, How to upgrade to macOS Sierra - Apple Support

How to upgrade to macOS High Sierra - Apple Support

and if your MacBook is from early 2015 you might be able to get Mojave,

How to upgrade to macOS Mojave - Apple Support

so come on give us information on your Mac and the OS it is running and we might be able to help you further.

As your Mac mini is from 2012 it can upgrade to a newer OS than El Capitan the App Store can see this and refuses you the download, your Mac mini can upgrade to High Sierra, How to upgrade to macOS High Sierra - Apple Support if it is a late 2012 Mac mini then it could upgrade to Mojave, How to upgrade to macOS Mojave - Apple Support

If you had downloaded El Capitan previously it should be available from the Purchased section of the App Store.

Get macOS High Sierra.

get El Capitan from the App Store

I upgraded my 2012 Macmini 6,1 with an SSD. I'm trying to install El Capitan (the OS on the previous hard drive), but I get a "Temporarily not available" SEM. Why can't I access the El Cap install?

Check the App Store Purchases tab and see if you can download it from there.

Your computer can run High Sierra, which can cause that error message.