Make sure you followed the installation steps carefully, especially the "build" part (https://cookiebadger.github.io/assetplacer-docs/installation.html). Notice the small build button next to the play button in Godot 4.2+.
If this does not help, remove the addon, but also remove the "mono" folder inside the ".godot" folder in your projects root directory. Then, add plugin again and build. It should work now.
Otherwise, try the plugin in a new project, to see if the issue is related to the project you are adding it to, and let me know your findings.