Please try to create a bookmark with following URL:

javascript:var r="html,body{overflow:auto !important;}"; var s=document.createElement("style"); s.type="text/css"; s.appendChild(document.createTextNode(r)); document.body.appendChild(s); void 0;

Open the page and simply click on the bookmark. This is basically what the add-on automates.
If this does work, there might be a configuration issue. If this doesn't help either, there is another reason why you can't scroll.

In my browser, https://old.reddit.com/ does have a scrollbar without my addon, probably this issue is caused by another addon or a configuration in your profile. You can try with a clean profile.