How to solve a catch 22 situation trying to upgrade an old 2009 mac mini to El Capitan with an AppleID created in 2019?
here's how I fixed that situation.


I downloaded Apple Music on my Android phone.
I start the procedure to subscribe to the service with my existing AppleID.
As it happened in my old Mac Mini, Apple Music on Android opened a popup saying my AppleID had not yet been verified.
On Android, however, in contrast with the Mac Mini, the verification link worked.
I had to retype the details of my credit card that I had already provided when I created the AppleID for the first time.
The verification process completed successfully, Apple debited my credit card for the second time, cancelling the payment after a while.
After that, I accessed the Apple Store from the old Mini Mac with the AppleID I had just verified on Android and...
It worked flawlessly. I downloaded OS X El Capitan and completed the upgrade successfully.


I write this procedure in case someone else will get stuck in the same, even highly unlikely, situation.