Exception driven development

clients_controller.rb

class ClientsController < ActionController::API
  def update_registration_number
    update_registration_number_service.(registration_number)
    render_success
  rescue UpdateRegistrationNumber::RegistrationNumberAlreadyExist
    render_error
  end
  # ...
end

update_registration_number.rb

class UpdateRegistrationNumber
  RegistrationNumberAlreadyExist = Class.new(StandardError)
  
  def call(registration_number)
    raise RegistrationNumberAlreadyExist if exist?(registration_number)
    # ...
  end
  
  # ...
end

https://gist.github.com/KamilLelonek/1ba97620ced153d05b6e#file-clients_controller-rb-L3

I believe you are missing the call to .call here.