cannot install grub while installing ubuntu

[na]

while installing ubuntu from live usb i am encountering an error that says Unable to install GRUB in /dev/sda

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubiquity 2.21.63.2 [modified: lib/partman/automatically_partition/question]
ProcVersionSignature: Ubuntu 4.4.0-31.50-generic 4.4.13
Uname: Linux 4.4.0-31-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CasperVersion: 1.376
Date: Sun Jan 21 21:14:34 2018
InstallCmdLine: file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd.lz quiet splash --- maybe-ubiquity
LiveMediaBuild: Ubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
ProcEnviron:
 LANGUAGE=en_IN
 PATH=(custom, no user)
 LANG=en_IN
 SHELL=/bin/bash
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)