Hi,
It is because you still do not config for WPBakery Page Builder plugin:
Go to WPBakery Page Builder > Role Manager > Select for the Post typs is Custom then tick on the boxes that will be the pages you want to edit with WPBakery Page builder: http://prntscr.com/o5vocc
I did it for you. And now you can edit your Footer Builder.
Regards,
Yame