Installation error of Ubuntu 18.04 AMD64 on HP Compaq 6200 Pro MT

[Alan AZZERA]

Error message (translated from French to English) : "the package grub-efi-amd64-signed cannot be installed on /target/, ... system won't boot"
It's the first time I see this kind of error. I use an USB key, maybe the key is defective ? I will try to verify the medium...

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14.6 [modified: lib/partman/automatically_partition/question]
ProcVersionSignature: Ubuntu 4.15.0-29.31-generic 4.15.18
Uname: Linux 4.15.0-29-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.2
Architecture: amd64
CasperVersion: 1.394
CurrentDesktop: ubuntu:GNOME
Date: Thu Jan 24 15:07:01 2019
InstallCmdLine: BOOT_IMAGE=(loop)/casper/vmlinuz root=UUID=0318-8EB4 maybe-ubiquity debian-installer/language=fr keyboard-configuration/layoutcode=fr keyboard-configuration/variantcode=oss iso-scan/filename=/ubuntu-18.04.1-desktop-amd64.iso boot=casper file=/cdrom/preseed/ubuntu.seed noprompt quiet splash --
LiveMediaBuild: Ubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180725)
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)