One of you’re other plugins is adding inline styles to all Elements, if you right click > inspect the site and check most of the elements you will see: style="height: auto !important;"
Whilst that style is there it will be impossible to set the inside-sidebar container to 100% height, which is required to make that element sticky.
You would need to remove what ever plugin that is causing that to make it work. And i cannot tell you which plugin is causing it.