# RuntimeError:
     #   Ransack needs ActionText::RichText attributes explicitly allowlisted as
     #   searchable. Define a `ransackable_attributes` class method in your `ActionText::RichText`
     #   model, watching out for items you DON'T want searchable (for
     #   example, `encrypted_password`, `password_reset_token`, `owner` or
     #   other sensitive information). You can use the following as a base:
     #   
     #   ```ruby
     #   class ActionText::RichText < ApplicationRecord
     #   
     #     # ...
     #   
     #     def self.ransackable_attributes(auth_object = nil)
     #       ["body", "created_at", "id", "locale", "name", "record_id", "record_type", "updated_at"]
     #     end
     #   
     #     # ...
     #   
     #   end

Solution:

# config/initializers/action_text.rb

ActiveSupport.on_load(:action_text_rich_text) do
  class ActionText::RichText < ActionText::Record
    def self.ransackable_attributes(auth_object = nil)
      authorizable_ransackable_attributes
    end
  end
end