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.
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.