why can't I reinstall el Capitan on my computer, has Apple deliberately made my Mac Mini into a door stop?

OK. Follow these instructions exactly:

When you get the error message, got to Utilities menu and then select Terminal. Then type:

date -u 0418171516

Now press the enter key.

Now type:

exit

.. and then enter again.

Now try to reinstall again; does it work this time? (If not, what happens?)

Somehow, I was able to get to the internet download screen after restarting and holding option command R at the beginning of each reset. Then it downloaded the new operating system.

What happens when you try to reinstall?

I get a message that says, "A required download is missing"

That’s an internet recovery install.

I’m glad to hear you managed to install the OS eventually :-)