How to reinstall MAC OS X El Capitan 10.11 ?

Hello, I've recently been given an iMac by a friend and I can't find a way to reinstall the OS on it, I have the same "this item is temporarily unavailable" message as everybody after typing my Apple ID. I know this is a frequent question and I looked tons of topics about it but nothing worked for me.

What I've tryed so far:

Put an older date then try to reinstall -> gives me a message that says "an error occured while preparing the installation. Try running this application again"

Booting an USB with El Capitan on it -> whatever keys i'm pressing after starting the mac it boots as usual and I can't launch the install from the USB

Clicking find help online and log into icloud on safari with the same Apple ID -> the icloud page never load but other pages would load if i'm typing another research

Starting the mac and holding cmd + opt + r or any other commands -> starts as usual, no globe, no Apple logo without loading bar, just the usual starting that brings me into the macOS Utilities

Erasing the hard drive -> no difference it's already empty anyway

So far I think this is all the solutions I could find on the differents topics. I tryed to put it as clear as I could but if anybody would need more information i'll do my best to provide them. I'm running out of hope and really wish someone could help me find a way out of this.