Comment 29 for bug 1851955

Ad suggested here

https://askubuntu.com/questions/1302806/grub-install-fails-because-of-efibootmgr-could-not-prepare-boot-variable-inva

run manual grub install - this helped:

sudo grub-install --target=x86_64-efi --bootloader-id=GRUB --efi-directory=/boot/efi --no-nvram --removable

Installing for x86_64-efi platform.
Installation finished. No error reported.