Ruby Google API Resource Not Found: Domain - ruby

Question

I've managed to fix the original issue below by removing the .readonly from the URL.
I'm now being presented with this error:
{"error"=>{"errors"=>[{"domain"=>"global", "reason"=>"notFound", "message"=>"Resource Not Found: domain"}], "code"=>404, "message"=>"Resource Not Found: domain"}}
Again, I've taken a look at this error online but all the links seem to go to an error page on Google and nothing to help determine a fix for this error.
Original Issue
I'm trying to use the below script to access to Google Directory API using Ruby but I'm receiving an error when I try to execute the script.
Script
#!/usr/bin/env ruby
require 'rubygems'
require 'json'
require 'google/api_client'
require 'active_support/core_ext/hash'
# The value of client_email from the JSON file goes here
SERVICE_ACCOUNT_EMAIL = 'xxxxxxxxx-bk7gbhr8k2dfvtidm0cc0msnipo34c8p#developer.gserviceaccount.com'
# This should be an Admin
ACT_ON_BEHALF_EMAIL = 'xxxxxx#gmail.com'
# File path to the certificate
SERVICE_ACCOUNT_PKCS12_FILE_PATH = '/home/xxxx/3ce8a60d43a8fa5cb747441fdfd90c3b91d6f45b-privatekey.p12'
key = Google::APIClient::PKCS12.load_key(SERVICE_ACCOUNT_PKCS12_FILE_PATH, 'notasecret')
asserter = Google::APIClient::JWTAsserter.new(SERVICE_ACCOUNT_EMAIL, 'https://www.googleapis.com/auth/admin.directory.user.readonly', key)
client = Google::APIClient.new(:application_name => 'GAAccountAudit')
client.authorization = asserter.authorize(ACT_ON_BEHALF_EMAIL)
api = client.discovered_api('admin', 'directory_v1')
result = client.execute(
:api_method => api.users.list,
:parameters => {
'domain' => 'http://www.xxxxxx.com',
'orderBy' => 'givenName',
'maxResults' => 500,
'fields' => 'users(id,etag,primaryEmail,name,suspended)',
}
)
users = JSON.parse(result.body, {:symbolize_names => true})[:users]
users.each do |u|
puts "#{u[:name][:fullName]} <#{u[:primaryEmail]}>"
end
Error
{"error" : "unauthorized_client","error_description" : "Unauthorized client or scope in request."}
I've read up on this error but I've not been able to find a conclusive answer to this. Could anyone please advise how I resolve this error?