Fatal error with Grub on installing Ubuntu with Win10 on dual boot basis

[C K Patel]

I got a fatal error while installing Ubuntu on a system where win10 was pre-existing. The error message has something to do with Grub

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: ubiquity 21.04.19
ProcVersionSignature: Ubuntu 5.11.0-16.17-generic 5.11.12
Uname: Linux 5.11.0-16-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu65
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.461
CurrentDesktop: ubuntu:GNOME
Date: Sat Jun 5 15:19:09 2021
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 21.04 "Hirsute Hippo" - Release amd64 (20210420)
ProcEnviron:
 LANGUAGE=en_IN
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_IN
 LC_NUMERIC=C.UTF-8
RebootRequiredPkgs:
 linux-image-5.11.0-16-generic
 linux-base
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)

[Ubuntu Foundations Team Bug Bot (crichton)]

Exception during installation:
Jun 5 09:48:57 ubuntu plugininstall.py: Traceback (most recent call last):
Jun 5 09:48:57 ubuntu plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 1872, in <module>
Jun 5 09:48:57 ubuntu plugininstall.py: install.run()
Jun 5 09:48:57 ubuntu plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 60, in wrapper
Jun 5 09:48:57 ubuntu plugininstall.py: func(self)
Jun 5 09:48:57 ubuntu plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 236, in run
Jun 5 09:48:57 ubuntu plugininstall.py: self.configure_bootloader()
Jun 5 09:48:57 ubuntu plugininstall.py: File "/usr/share/ubiquity/plugininstall.py", line 994, in configure_bootloader
Jun 5 09:48:57 ubuntu plugininstall.py: raise install_misc.InstallStepError(
Jun 5 09:48:57 ubuntu plugininstall.py: ubiquity.install_misc.InstallStepError: GrubInstaller failed with code 1
Jun 5 09:48:57 ubuntu plugininstall.py: