Hello,
unfortunately, nothing David suggested worked, so I finally set it on in the bios Legacy (via F12) and then reinstalled Debian.
Now if I turn on, Grub appears with Debian that starts (even if it takes a while to boot) and if I want to use Windows, at boot, I have to click F12 and select boot with UEFI and then Windows 10 starts.
I tried to install os-prober on Debian, but it is already installed and doing update-grub, but Windows is not shown (Probably because it is inserted in the UEFI boot).
Is there any way to fix this mess?
Francesco