ubuntu hirsute install; to sda1 with grub on sda - fails to install (BIOS box; no EFI)

Simple install of ubuntu hirsute daily (QA-test)
single partition only used; sda1 of
- dell [optiplex] 780 (c2q-q9400, 8gb, amd/ati cedar radeon hd 5000/6000/7350/8350)

install to /dev/sda1
boot loader was reported as installing to /dev/sda

Apr 14 01:33:23 ubuntu grub-installer: Installing for x86_64-efi platform.
Apr 14 01:33:35 ubuntu grub-installer: grub-install: error: cannot copy `/usr/lib/grub/x86_64-efi-signed/grubx64.efi.signed' to `/boot/efi/EFI/ubuntu/grubx64.efi': No space left on device.
Apr 14 01:33:35 ubuntu grub-installer: error: Running 'grub-install --force --target x86_64-efi "/dev/sda"' failed.

system is BIOS and not a uEFI device..

ProblemType: Bug
DistroRelease: Ubuntu 21.04
Package: ubiquity 21.04.14
ProcVersionSignature: Ubuntu 5.11.0-13.14-generic 5.11.7
Uname: Linux 5.11.0-13-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu62
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.461
CurrentDesktop: ubuntu:GNOME
Date: Wed Apr 14 11:39:18 2021
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 21.04 "Hirsute Hippo" - Beta amd64 (20210413)
ProcEnviron:
 LANGUAGE=en_AU.UTF-8
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_AU.UTF-8
 LC_NUMERIC=C.UTF-8
RebootRequiredPkgs:
 linux-image-5.11.0-13-generic
 linux-base
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)