fallo la instalacion del grub en /dev/sda

Instale windows 10 pro en el hd sda en 200 gb de disco y quedo reiniciando bien. Luego instale ubuntu 20.04 y fallo la instalacion con el error fatal "fallo grub install on /dev/sda". luego repeti la operacion con ubuntu 20.10 y salio el mismo error.

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: ubiquity 20.10.10
ProcVersionSignature: Ubuntu 5.8.0-20.21-generic 5.8.10
Uname: Linux 5.8.0-20-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu48
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.452
CurrentDesktop: ubuntu:GNOME
Date: Thu Oct 15 22:23:34 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 20.10 "Groovy Gorilla" - Beta amd64 (20200930)
ProcEnviron:
 LANGUAGE=en_US.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 LC_NUMERIC=C.UTF-8
RebootRequiredPkgs:
 linux-image-5.8.0-20-generic
 linux-base
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)