In which folder did you install the script?
This particular script (it’s the only Id-Extras like this for now), needs to be installed in the “startup scripts” folder.
To get to that folder, in InDesign, in the Scripts Panel, right-click on the folder called “Application” and select “Reveal in Explorer/Finder”.
When you do that, you’ll see a folder called “startup scripts”, and that’s where Always Fit Spread goes. (See the attached screenshot.)
I don’t think this issue has anything to do with Catalina.
Let me know how it goes!