I tried installing Zorin OS 18 using a different USB (which I had previously flashed with balenaEtcher when I had Windows) and ran into a similar error:
Unable to install GRUB in /dev/nvme0n1
Executing ‘grub-install /dev/nvme0n1’ failed.This is a fatal error.
Update: It didn’t occur to me till then to just restart the PC and see what happens. It appears Zorin OS installed but didn’t remember setup details (such as user credentials) I provided during installation, but after the first reboot it seemed fine. So I re-installed Ubuntu 25.10 again with more or less the same outcome. It only remembered my LUKS password but I had to repeat much of the same setup process by selecting a time zone, creating an account, etc.
Data seems to have persisted since then so I’m not sure if this GRUB issue is causing any other underlying problems, but for now it’s sort of working? Except that I am having issues with checking for updates and updating firmware, but I assume I should create a separate thread for that.
Just checked and UEFI Fast Boot is enabled and Ubuntu is the first and only boot option. I’m not sure if I should bother disabling it now that Ubuntu seems to be installed (though I’m having firmware update issues) but I’m open to suggestions if you think it’s relevant to my current situation.
I mentioned the other details in the OP but to reiterate: I was looking to overwrite my Windows 10 installation which was encrypted with BitLocker. I’m not looking to dual boot now but I attempted it in the past with Fedora + W10 and it went terribly wrong. I only mentioned that history as it seems like that must’ve been the initial cause of this UEFI/bootloader issue I’m having.