Comment 2 for bug 1869278

The details in the installer syslog show:

Mar 26 23:53:05 ubuntu grub-installer: Instalando para plataforma x86_64-efi.
Mar 26 23:53:06 ubuntu kernel: [ 6272.712620] FAT-fs (sda1): error, fat_get_cluster: invalid cluster chain (i_pos 0)
Mar 26 23:53:16 ubuntu grub-installer: grub-install: error: no se puede abrir «/boot/efi/EFI/ubuntu/grubx64.efi»: Error de entrada/salida.
Mar 26 23:53:16 ubuntu kernel: [ 6282.150719] FAT-fs (sda1): error, fat_get_cluster: invalid cluster chain (i_pos 0)
Mar 26 23:53:16 ubuntu grub-installer: error: Running 'grub-install --force "/dev/sda"' failed.

It looks like the installation failed because your EFI System Partition, /dev/sda1, which is a FAT filesystem shared between Windows and Ubuntu, has corruption. You can try rebooting to Windows and making sure you shut the system down cleanly, then try to reinstall, and see if that fixes the problem. Or, from the Ubuntu live environment, you can try running these commands from a shell:

  sudo umount /boot/efi
  sudo fsck.vfat /dev/sda1
  sudo mount /boot/efi

and then retry the installation to see if it succeeds.

The 'fsck.vfat' command may ask you for permission to make changes to the filesystem to correct the errors that it finds. Please note that, depending on the nature of the corruption it detects, this has the potential to make your Windows system unbootable afterwards.