Testing Adwords Api with Service Account

I am using http://ift.tt/2mwfMCG gem for using adwords api for OfflineConversionFeedService. Code looks alright:

module AdwordsHelper
def mark_conversion_in_ads(customer)
begin
  config_filename = File.join(Rails.root, 'config', 'adwords_api.yml')
  adwords = AdwordsApi::Api.new(config_filename)
  conversion_feed_srv =
      adwords.service(:OfflineConversionFeedService, :v201702)
  conversion_time = Time.new.strftime("%Y%m%d %H%M%S")
  conversion_value = '100'.to_f

  feed = {
      :google_click_id => customer.gclid,
      :conversion_name => 'LeadCreatedAll',
      :conversion_time => conversion_time,
      :conversion_value => conversion_value,
      :conversion_currency_code => 'GBP'
  }
  return_feeds = conversion_feed_srv.mutate([
                                                {:operator => 'ADD', :operand => feed}])
  return_feeds[:value].each do |return_feed|
    puts ("Uploaded offline conversion value %.2f for Google Click ID '%s', " +
             'to %s') % [return_feed[:conversion_value],
                         return_feed[:google_click_id],
                         return_feed[:conversion_name]]
  end


rescue AdwordsApi::Errors::ApiException => e
  puts e.backtrace
  Logger.log_message("Exception occurred: %s\n%s" % [e.to_s, e.message])
rescue AdsCommon::Errors::OAuth2VerificationRequired => e
  puts "Authorization credentials are not valid. Edit adwords_api.yml for " +
           "OAuth2 client ID and secret and run misc/setup_oauth2.rb example " +
           "to retrieve and store OAuth2 tokens."
  puts "See this wiki page for more details:\n\n  " +
           'http://ift.tt/1di4CsR'

  # HTTP errors.
rescue AdsCommon::Errors::HttpError => e
  puts "HTTP Error: %s" % e

    # API errors.
rescue AdwordsApi::Errors::ApiException => e
  puts "Message: %s" % e.message
  puts 'Errors:'
  e.errors.each_with_index do |error, index|
    puts "\tError [%d]:" % (index + 1)
    error.each do |field, value|
      puts "\t\t%s: %s" % [field, value]
    end
  end
end

end

end

My adwords_api.yml file is:

---
:authentication:
:method: OAUTH2_SERVICE_ACCOUNT
:oauth2_keyfile: "/path/to_server_account_file/jsonkey.json"
:developer_token: test_level_developer_token_of_production_manager_account
:client_customer_id: client_customer_id_of_test_manager_account
:oauth2_token:
  :access_token: access_token_generated_setup_oauth.rb
  :issued_at: 2017-03-16 13:12:25.005441000 +05:30
  :expires_in: 3600
  :id_token: 
 :service:
  :environment: PRODUCTION
:connection:
  :enable_gzip: false
:library:
 :log_level: INFO

I have been trying to test this setup for a long time and am getting this error: Exception occurred: [AuthenticationError.NOT_ADS_USER @ ; trigger:''] [AuthenticationError.NOT_ADS_USER @ ; trigger:'']

I have tried a lot of links: managing test accounts

Can someone please provide exact steps on how to test it out. For some more clarification, I have Test Account developer key in adwords.

Using adwords api v201702 The api I am trying to use is OfflineConversion