**** Thanks to all who replied, or would! *****
I have built up a theme on Canvas 6 HTML theme on Bootstrap 4 here, on the latest C5 v8.
Everything is groovy, except "Add Layout" does not work. If I add a layout to an area, the blue spinner bar never finishes, the add layout UI never renders completely, and in the Inspector console I find this: TypeError: undefined is not a function (near '...a.widget...'), and that is pointing into the jquery file. I have not found anything else in the site or backend that does not work.
I have built at least a couple dozen C5 sites from back in 5.4 days, and several on v8, and I did another Bootstrap 4 one from a different HTML theme that works correctly. Self-taught web guy, I never learned to debug code.
I've checked all the normal gotchas I can think of, I'm not running two Jquery's for instance.
I can share access to the FTP and site.
This is a paid task.
[edited post job to remove client-specific information]
If I reverse this, so that I load C5's JQ, I get the add layout interface, but nothing on it works.
Also, I am loading BS3 for the layout grid... figured I'd just hack the classes if I could get Layout up and running...
It was a good build, and thankfully I got stuck, or never would have known all this.
I can help, please check your PM.