'the 'grub-efi-amd64-signed' package failed to install into /target/' during Linux Mint installation on an iMac
Took me weeks, but finally solved it for my case.

My install would go well, but the installation of grub would fail with the same message because of corrupted data in the EFI partition.

Fastest fix was to run a windows chkdsk to fix the partition, then the next install went fine.