If you encounter the "Config file not found." message whilst trying to view a gallery locally in Safari 11, then please see this forum post for a full explanation and a workaround.
Unfortunately, Safari 11 (released 19 September 2017) introduced a new security restriction which prevents local viewing of Juicebox galleries by default. This issue is specific to viewing galleries locally (from your computer's hard drive) and there is no such issue once the galleries have been uploaded to a web server (where they can be viewed in any modern browser).
Selecting 'Disable Local File Restrictions' from Safari 11's 'Develop' menu (full instructions in the link above) should, once again, allow you to view your galleries locally in Safari 11.
If you are not using Safari 11, then please see this FAQ for more general information on the "Config file not found." message:
When I view my gallery I see the message 'Config file not found'. How do I fix this?
I hope these notes help.
However, if you continue to experience difficulties, please let me know and I'll try to help further.
Let me know if you encounter the error whilst trying to view a gallery locally or after the gallery has been uploaded to your web server. Also, let me know what browser(s) you see the message in and, if your gallery has been uploaded to your web server, please post a link to the gallery so that I can see the problem for myself.
Once I'm able to see the gallery on your web server, I should hopefully be able to determine the cause of the problem and propose a solution.