The substitute server at <https://mirror.hydra.gnu.org> has been
unreachable since at least today 3PM CET, 7 hours ago¹.

We are investigating and will send a notification as soon as the service
is back up.

In the meantime, you can use a different substitute server such as
<https://berlin.guixsd.org/>.  To do that you first need to authorize it:

  https://gnu.org/s/guix/manual/en/html_node/Substitute-Server-Authorization.html

and then pass ‘--substitute-urls=https://berlin.guixsd.org’ as an option
to the ‘guix’ commands:

  https://gnu.org/s/guix/manual/en/html_node/Common-Build-Options.html#client_002dsubstitute_002durls

Apologies for the inconvenience.

Ludo’.

¹ This was first reported at <http://issues.guix.info/issue/33539>.