Please make sure that you use the latest version of Visual Composer and Ultimate Addons for Visual Composer https://www.8theme.com/download-plugins/page/2/
Also, try to activate Storefront theme – official WC theme and check if you have this issue with our theme only or even with official theme too. In case, problem appears only with our theme try to disable 3-rd party plugins (means the plugins that are not included in the theme package are not in the list of required plugins) and check your issues.
Plugins can be deactivated not through ftp but trough wp-admin->dashboard->plugins->check all plugins and select “deactivate”. Then you will get notification on the top of the screen with necessaty to enable required plugins. You will be able to check if theme working fine with default plugins. On next step you may enable all 3-d party plugins one y one and check which of them causing misconfiguration.
It is no error, simply refresh the page or enter your FTP password.
The problem with variable product caused by older versions Visual Composer and Ultimate Addons for Visual Composer plugin versions. To update premium plugins, connect to your server using FTP. Copy extracted plugins folders into wp-content/plugins/. The latest versions of plugins you can find here.