boot-repair is asking for a repo to install grub-efi-amd64-signed

...but I've no idea what repo it needs.

I'm running boot-repair from the Ubuntu 20.04 live USB stick, have successfully installed boot-repair and see this every time I run it:

Please enable a repository containing the [grub-efi-amd64-signed] packages in the software sources of Ubuntu 20.04.2 LTS (mapper/vgubuntu-root). Then try again.

I'm not clear on a couple of points here (and would very much appreciate help). 1. Where do I find such a repository? and, 2. If this grub package is essential to boot-install, why isn't it included in the boot-install live USB (which I've also tried, with the same result)?

The first question is practical; the second more philosophical. If somebody can put my mind to rest over either or both of these I'd be very grateful. And if more information is needed I'll be happy to follow up here.

Meanwhile, current boot details here: https://paste.ubuntu.com/p/MtfxvwwV6W/

LATER

I've also tried installing grub-efi-amd64 independently of boot-repair, using the Synaptic Package Manager. This doesn't seem to impress boot-repair in the least---it still insists on running the install itself from a repo whose location it teasingly refuses to reveal.

-- Chris