An older bug ( discusses a similar issue, but for a foreign distro with Guix installed (not a native Guix distribution).

That bug mentions two things:

- make sure that either ‘glibc-utf8-locales’ or ‘glibc-locales’ is installed (as root)
- make sure that the GUIX daemon is configured to use a UTF locale so it can handle the UTF-encoded filenames in the nss-certs package.

I'm not sure whether these issues apply to a native Guix distribution. What I do know is that when I encountered the error myself (running Guix on Debian 10) I needed one additional thing: the environment of the user installing the package had to include a UTF locale. After switching this environment from LANG=C to LANG=en_US.utf8, the package installed without issue.