"grub-efi-amd64-signed package failed to install into /target/' error."
Warning if you have just downloaded Mint 19 and or Ubuntu latest versions when attempting to install you may get the following error. Installation is OK up to boot loader install, but fails with "'grub-efi-amd64-signed package failed to install into /target/' error." This is happening with Mint19 and Ubuntu.
The fix for now if you encounter this problem is to do the install without being connected to the Internet. Once installed you can reconnect and download updates. The downside is you won't be able to get the updates during the install and you will have to download the codecs that you need. a couple of extra steps, but it works.
The problem is Ubuntu and the signing keys. Since Mint uses Ubuntu's Repos and signing keys that is where the problem is. Its not Mint.