There have been some issues with the official substitute server
(that are being investigated, though perhaps this is a separate issue)
you might want to consider first installing a rather minimalistic system,
without the CUPS service (I assume that's the reason hplip-minimal is
Then, reboot into the installed system and gradually add back system
services (such as CUPS and a graphical environment) etc. If some
substitutions are still failing, you could add the "--no-substitutes"
option to "guix system reconfigure" to build these locally.
Perhaps --fallback would work as well; if it does, perhaps that should
be done in the installer by default if it isn't already.
p.s. Text is preferred over image attachements.