while installing ubuntu 16.4 tere is an error to install grub boatloader

[Silke Teuven]

I installed Ubuntu over an USB Stick and next to windows, I always get the error that Grub boatloader is unable to be installed in dev/sda

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubiquity 2.21.63.4 [modified: lib/partman/automatically_partition/question]
ProcVersionSignature: Ubuntu 4.10.0-28.32~16.04.2-generic 4.10.17
Uname: Linux 4.10.0-28-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.10
Architecture: amd64
CasperVersion: 1.376.2
Date: Mon Oct 30 17:24:31 2017
InstallCmdLine: file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd.lz quiet splash --- maybe-ubiquity
LiveMediaBuild: Ubuntu 16.04.3 LTS "Xenial Xerus" - Release amd64 (20170801)
ProcEnviron:
 LANGUAGE=de_DE.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)