On moving a .html template to a wordpress one, I found this "popper required" popping up regularly :)
Two reasons it happened for me: and the console error can be deceptive:
The error in the console can send you down the wrong path. It MIGHT not be the real issue. First reason for me was the order in which you have set your .js files to load. In html easy, put them in the same order as the theme template. In Wordpress, you need to enqueue them in the right order, but also set a priority if they don't appear in the right order,
Second thing is are the .js files in the header or the footer. Moving them to the footer can solve the issue - it did for me, after a day of trying to debug the issue. Usually doesn't matter, but for a complex page with lots of js libraries, it might!