I just tested as well and can confirm that this plugin no longer works.
@techAPJ is it possible this plugin of yours is outdated, now that user settings have moved to the user options table? This is not a priority for me right now - perhaps you or @Jared_Needell could update the plugin.
Note how in the file there is no method defined find_by, there is find_by_slug and find_by_email.
That is because find_by is built into ActiveRecord, and you are querying the category table for any record containing slug = 'whatever value you asked for', which is inherently different than using the method that exists in the category.rb file.
I have installed your plugin, but it doesn’t seem to work…
I’ve followed instruction here Install a Plugin
After installation Discourse works fine, but it seems nothing changed: if I look at users’ profiles of the group I wanted to force to watch a category, nothing changed, the category doesn’t appear among the watched ones.
The job runs once a day, if you look in sidekiq (http://discoursever/sidekiq) under scheduler, you can manually trigger a task. If you want the script to run more frequently, just update the frequency in the script.