grub fatal error during installation

installing alongside windows 10 on an mbr dell xps 1640. reports /dev/sda fatal error.

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: ubiquity 20.10.11
ProcVersionSignature: Ubuntu 5.8.0-23.24-generic 5.8.14
Uname: Linux 5.8.0-23-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu50
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.452
CurrentDesktop: ubuntu:GNOME
Date: Fri Oct 16 20:52:30 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 20.10 "Groovy Gorilla" - Beta amd64 (20201016.1)
ProcEnviron:
 LANGUAGE=en_CA.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_CA.UTF-8
 LC_NUMERIC=C.UTF-8
RebootRequiredPkgs:
 linux-image-5.8.0-23-generic
 linux-base
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)