No se puede instalar el grub en ubuntu 18

[luis]

Me salio un mensaje en la instalacion de Ubuntu 18 que decia q no se pudo instalar GRUB, antes de instalar bootee el usb con el creador de particiones de ubuntu 16 y me boto un error. Pero a pesar de eso continue con la instalacion.

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: ubuntu:GNOME
Date: Sat May 19 02:02:33 2018
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed boot=casper only-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
ProcEnviron:
 LANGUAGE=es_PE.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=es_PE.UTF-8
 LC_NUMERIC=C.UTF-8
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)

[Phillip Susi (psusi)]

Your installation medium is corrupt; you will need to recreate it and may want to verify it using the check disc option at the boot menu.