I had Windows 8 and Ubuntu 16.04 and after making a clean install for Ubuntu 18.04 on the 16.04 partition, I got the message: grub-efi-amd64-signed' package failed to install into /target/.
at the end of the installation and nothing boots up now.
This is how GParted looks.
gparted
and include a screen shot of what your HDD now looks like in gparted? – WinEunuuchs2Unix May 12 '18 at 14:34