For some kind of actions to work on production (for instance, sending emails on user registration) you need to configure an ActiveJob backend on your application.

If you do not want to introduce any other external dependencies for decidim (such as redis), you can use the delayed_job backend. That setup only depends on an underlying database which is already a requirement for decidim to work. Add the delayed_job gem to your Gemfile and follow the instructions in its readme to set it up.

However, Decidim is agnostic on which kind of backend do you use, so feel free to set up the backend (ie sidekiq, resque, etc) you like the most.