This problem is covered in one issue discussion on GH, the .iso file grew larger than 2GB and thus does not fit within GitHub asset limits anymore. You can checkout the repo, and with local guix (package manager version fine) can rebuild the .iso yourself. Takes quite a bit of time though.
The magic is that the build-iso.sh script needs to run in a guix system as opposed to a Debian system with the guix package manager installed.
I was able to use the guix package manager in Debian to install the nonguix system on an external ssd. After booting the nonguix system on the external ssd the build-iso.sh script finishes with success.
Next problem is that the installation from that iso still installs the limited gnu kernel.
So far it seems that a successful approach is to enable nonguix on the debian system and install from there into another partition. Without the help of an installer you need to generate a config.scm yourself.
I can’t find the error message, but it said something about a locked sqlite file.
The same error showed when I used both the nonguix iso ( Nonguix / nonguix · GitLab ) and the system crafters iso.