Hi!

Did you authorize the substitute server on the client?

From the manual (Invoking `guix publish'):

>    The signing key pair must be generated before ‘guix publish’ is
> launched, using ‘guix archive --generate-key’ (*note Invoking guix
> archive::).
> ...
>    Once a publishing server has been authorized (*note Invoking guix
> archive::), the daemon may download substitutes from it:

Hope that helps!

-- 
Pierre Neidhardt
https://ambrevar.xyz/