the grub-efi-amd64-signed package failed to install

I came across this error whilst installing Elementary OS 5.1 onto a 2011 MacBook. After checking out why, it became obvious. The MacBook boots in UEFI mode and not MBR. Normally I create my Linux USB drives with Balena Etcher and trust it to get the drive right. A quick switch to RUFUS and create the USB drive with GPT (UEFI-Non CSM) and hey presto the issue went away.

This is because Etcher did not create a true UEFI bootable drive, this was evident by the fact that when you attempt to boot from the USB on the MacBook (by holding the ALT key down on startup) there were 2 EFI boot options.

Hope this helps someone if they come across this error.