GRUB installation failed - installing Ubuntu Budgie 18.10 on USB issue

[Michel]

While trying to install Ubuntu Budgie on a USB i got the following error message:
"GRUB installation failed: The 'grub-efi-amd64-signed'package failed to install into / target/. Without the GRUB boot loader, the installation system will not boot".

kindly need your help to solve this issue.

Thank you

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.14
ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
Uname: Linux 4.15.0-20-generic x86_64
ApportVersion: 2.20.9-0ubuntu7
Architecture: amd64
CasperVersion: 1.394
CurrentDesktop: Budgie
Date: Fri May 4 08:39:17 2018
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu-budgie.seed boot=casper only-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu-Budgie 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
ProcEnviron:
 LANGUAGE=en_US.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 LC_NUMERIC=C.UTF-8
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)