I solved it !
Since the backbone of Qubes is based on Fedora, while I had always installed Debian-based systems, I thought the problem might be the different way of managing partitions or something, so I took a tour of the Fedora site to try to see if with that iso I would at least get to the installer…
On the site I saw the Fedora Media Writer that creates the bootable pendrives, so I tried it and the Fedora installation pendrive so created showed me the installer without problems!
So this time I created the installation pendrive using Fedora Media Writer and the Qubes iso…
And it worked!
To test it all without erasing my Laptop’s internal SSD (in case something goes wrong) I installed Qubes in an external pendrive (for now) and it works pretty well.