Hmmm… another plugin is adding inline styles to all your elements: ie. height: auto !important; which is making it impossible to change the inside-sidebar container to 100% which is required to make the element sticky.
If you Right Click > Inspect the page you will see this for example:
<div id="page" class="site grid-container container hfeed" style="height: auto !important;">
The inline style is being added by another plugin.
You would need to find which plugin is causing that by disabling each one at a time and check to see if the style is removed.
I would not recommend having the Gutenberg plugin installed, it is effectively in permanent Beta mode, so its great for testing out new features before they are added to WP core – but with that comes bugs.