What to do if Lubuntu won't load GRUB?

Question

linux

During installation, lubuntu comes to "installing grub" and throws out "Failed to install package grub-efi-amd64-signed to /target/". What to do? I install in "/dev/sda7", mount point "/".
On 1 forum it is written that "We need a /efi/boot partition formatted with fat12\16\32 and ranging in size from 100mb to 500mb (in fact, a negligible part of the partition is used)" - Where should I create this partition? Thanks in advance.