Comment 4 for bug 1905522

I'm seeing something similar on an ASUS desktop PC (AMD socket FM2, DDR3) which already has a few dual-boot options installed. I'm trying to install groovy 20.10 using Ubiquity from the live CD/DVD/USB. The installer goes through most of its paces, until it tries to install GRUB. It then dies with a pop-up box:

Error installing grub-efi-amd64-signed

installed grub-efi-amd64-signed package post-installation script subprocess returned error exit status 1

Attempting to install the package manually from a live desktop terminal window also fails, with:

$ sudo su -
# chroot /target
# apt install grub-efi-amd64

giving:

Setting up grub-efi-amd64 (2.04-1ubuntu35.1) ...
Installing grub to /boot/efi.
Installing for x86_64-efi platform.
grub-install: warning: Cannot read EFI Boot* variables.
grub-install: warning: read_file: could not read from file: Input/output error.
grub-install: warning: vars_get_variable: read_file(/sys/firmware/efi/vars/Boot000C-8be4df61-93ca-11d2-aa0d-00e098032b8c/raw_var) failed
: Input/output error.
grub-install: warning: efi_get_variable: ops->get_variable failed: Input/output error.
grub-install: error: failed to register the EFI boot entry: Input/output error.

The workaround seems to be, while the installer is stopped at the error message box but still open, launching a terminal window from the live desktop and mounting efivarfs manually. At that point, apt may be used to install the package, install grub, apt upgrade everything to the current release... whatever.

 sudo su -
 chroot /mount/target
 sh -c "mount -t efivarfs efivarfs /sys/firmware/efi/efivars; grub-install; update-grub;
 apt install --reinstall grub-efi-amd64-signed

After that, I was able to reboot into Ubuntu 20.10 (groovy) from the internal SSD.

Without that, I could try the installer any number of times and it would always fail the same way.

bugs.debian.org/cgi-bin/bugreport.cgi?bug=933523 looks to be related

bugs.launchpad.net/ubuntu/+source/grub2-signed/+bug/1874315 might be as well?