If you encounter the "Config file not found." message whilst trying to view a gallery locally (from your computer rather than from your web server) 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 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 this does not describe your problem, then more general information on the "Config file not found." message can be found in this FAQ:
When I view my gallery I see the message 'Config file not found'. How do I fix this?
I hope my notes above help.
Please let me know how you get on and if I can be of any further assistance.
If you encounter the "Config file not found." message in an online gallery, then please post the URL to your gallery's web page so that I can see the problem for myself and hopefully help further.
Thank you.