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.

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?

Please see this forum thread. Hopefully the links and suggestions will help.
(Deleting the ~/Library/Application SupportAdobe/AIR/ELS/JuiceboxBuilder-Pro folder has certainly helped other users experiencing a similar problem but maybe the other suggestions in the forum thread will help.)

That's great! Thank you for letting me know.

Selecting "View in Browser" should open your gallery's 'index.html' file in your system's default browser.
Please check that Safari 11 is your system's default browser by following the instructions on this Apple support page.
Also, check your system's file association for HTML files. It sounds like HTML files are currently associated with your text editor instead of Safari 11. Please check out this Apple support page for details, specifically the "Permanently change the app used to open a file" section.

Even though your system tells you that Safari is the default browser, try changing it to something different, e.g. Chrome or Firefox (and check the JuiceboxBuilder-Pro 'View in Browser' functionality after doing so), and then change it back to Safari.
I'm not sure if this will help but it's certainly worth a try.

I'm sorry to hear that my suggestions have not helped.
I'm really not sure what might be causing your problem if it is not a default browser or file association issue.
If I think of anything else that might help, I'll be sure to post back.
In the meantime, the best course of action is to deselect the 'View in Browser' button and just double-click the 'index.html' file inside the gallery folder (as you are already doing).