Cause: Probably visual composer is not enabled for your user role. Even visual composer plugin is activated, you need to set permission for which roles can access visual composer editor.
To do that navigate to Visual Composer > Role Manager under user roles for example Administrator, choose post types that you want to enable visual composer for. It is important to choose for “Pages” in order to edit pages via visual composer editor.
FYI: Visual composer frontend editor is not available with this theme.
If you still can’t edit via visual composer feel free to contact us by opening a support ticket.