GRUB installation fails at the end on /dev/sda in fresh install (erase old /)

[Benoît Rouits]

I had a Previous upgraded Ubuntu and for some reason, I decided to install a fresh one, getting /home (/dev/sdb1) non formatted and / (/dev/sda1) reformatted byt the partition manager at install time. At the end of installation, grub install fails on /dev/sda (popup) and I'm asked to restart anyway. I didn't, to be able to filling this bug. Now I will restart and try to figure out how to fix it.

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: ubiquity 20.10.13
ProcVersionSignature: Ubuntu 5.8.0-25.26-generic 5.8.14
Uname: Linux 5.8.0-25-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu50
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.455
CurrentDesktop: ubuntu:GNOME
Date: Sat Oct 24 18:01:13 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 20.10 "Groovy Gorilla" - Release amd64 (20201022)
ProcEnviron:
 LANGUAGE=fr_FR.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 LC_NUMERIC=C.UTF-8
RebootRequiredPkgs:
 linux-image-5.8.0-25-generic
 linux-base
 linux-base
 linux-base
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Benoît Rouits (brouits)]

I finally could re-install it by setting EFI ON in my BIOS, and creating an 50Mo EFI partition with the ubuntu installer. So, Ubuntu seem not to work anymore with old PC having not UEFI (or disabling it) ?