I tried installing the extension from the store but it doesn't seem to work at all. I am on Pop!_OS 20.10, GNOME 3.38.3.
Spotify recently updated their desktop client with a new design, although I'm not sure if this is what broke it since I did not discover this extension until a few days ago. Thus I was not able to test the extension before the Spotify redesign.
Please let me know what other information you need to debug this, I would be glad to help.