cannot install grub-efi-amd64-signed software package to /target while install Ubuntu 18.04

[jundong.wu]

While I was installing Ubuntu 18.04, I set the boot/ partition and EFI/ partition, but don't know why this notice came out.I tried everyway and they did not work.So, is there anyone can help me?

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: ubiquity 18.04.12
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
Date: Mon Jul 2 01:28:42 2018
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed boot=casper quiet splash ---
LiveMediaBuild: Ubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180425.1)
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)