Installation of Ubuntu 18.04 LTS crashed: bootloader could not be installed

[Thomas]

Hi,
I tried to install ubuntu 19.10 (by booting on a usb stick formatted by unetbootin) which failed (could not install bootloader). I then tried installing ubuntu 18.04 LTS, which failed in the same way. When I reboot, I come to a rescue grub shell.
Thanks for the help
Tom

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14.12
ProcVersionSignature: Ubuntu 5.0.0-23.24~18.04.1-generic 5.0.15
Uname: Linux 5.0.0-23-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.7
Architecture: amd64
CasperVersion: 1.394
CurrentDesktop: ubuntu:GNOME
Date: Sat Dec 28 10:24:55 2019
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed boot=casper only-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 18.04.3 LTS "Bionic Beaver" - Release amd64 (20190805)
ProcEnviron:
 LANGUAGE=fr_FR.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
 LC_NUMERIC=C.UTF-8
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)