This question isn't a duplicate of Ubuntu 16.10 Dual Boot error – 'grub-efi-amd64-signed' package failed to install. The question is exactly the same because the error is the same but it has a different solution.
Never had problems installing Ubuntu. I already installed Ubuntu on this machine before but on a hard disk. However, this is the first time trying to install it on an SSD. The first thing that seems wrong is that the installer doesn't recognize current OS. So I shrank my windows 10 partition using windows and then tried to install Ubuntu on the free space created. Almost at the end of the process, it gives me the error:
The 'grub-efi-amd64-signed' package failed to install into / target/. Without the GRUB boot loader, the installed system will not boot.
The bootable USB with Ubuntu Live boots only with CSM disabled.
If you need other information to figure out what's happening, ask in the comment section and I'll edit the post to provide it. Keep in mind that I cannot use Windows anymore now.
I followed some of the suggestion in the post linked in the comment but
boot-repair doesn't work. This is the log of boot-repair (sorry for the external link but it-s too long to post here).
As said in the comments, Windows is in BIOS boot mode. I created the bootable USB with rufus with the MBR (in theory should work with both BIOS and UEFI). However the bootable USB boot only with CSM disabled. With CSM enabled the error is:
Failed to open \EFI\BOOT\mmx64.efi - Not Found Failed to load image \EFI\BOOT\mmx64.efi: Not Found Failed to start MokManager: Not Found
I found no way to use the bootable usb with CSM enabled so I decided to change the way Windows boots from MBR to GPT with mbr2gpt. It seems that all went well. However, now I'm not able to boot with the bootable USB neither with CSM enabled nor disabled.