ubuntu hirsute desktop install to bios box; single ext4 partition only (replace system no format)

Ubuntu hirsute desktop install to
- hp dc7700 (c2d-e6320, 5gb, nvidia quadro nvs 290)

disk was partition prior to starting ubiquity
MBR/dos partition table with single partition using all disk space.

manual partitioning QA-test. I selected the SDA1 partition for install but did NOT format it (it was empty having been formatted only a minute before).

FYI: Simulating replacing an existing partition (on most of my systems I already have four primary partitions used), but this is a box I use for QA-testing so I don't care about loosing data here

ubiquity failed with grub-install failure..
system failed to boot, ending in emergency mode

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: ubiquity 21.04.17
ProcVersionSignature: Ubuntu 5.11.0-14.15-generic 5.11.12
Uname: Linux 5.11.0-14-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu65
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.461
Date: Mon Apr 19 15:28:12 2021
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 21.04 "Hirsute Hippo" - Beta amd64 (20210418)
RebootRequiredPkgs:
 linux-image-5.11.0-14-generic
 linux-base
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)