oh…
The problem is solved. I don’t know the exact reason, but after adding the search icon to the main menu in Enfold Child Theme Options’s main menu, I refreshed and ‘accordion’ worked fine. The reason I added the search icon is that when I tested ‘accordion’ on the test page, I tested ‘search’ together, and since then ‘accordion’ didn’t work properly.
And ‘accordion’ works fine even if I remove the search icon from the main menu of the enfold child theme option.