AuthenticationError.NOT_ADS_USER

[Travis]

I've followed the directions best I could and when I run the following test 
command I get this error:

    Fault message: [AuthenticationError.NOT_ADS_USER @ ; trigger:'<null>']

Here is how I'm authenticating (JSON key parsed via `OpenSSL::PKey::RSA`):

    json = JSON.parse(Rails.application.secrets.adwords[:oauth2_key])
    key = OpenSSL::PKey::RSA.new(json['private_key'])

    @client = AdwordsApi::Api.new({
      authentication: {
        method:             'OAUTH2_SERVICE_ACCOUNT',
        user_agent:         'AdManager/1.0.0',
        oauth2_key:         key,
        oauth2_issuer:      
Rails.application.secrets.adwords[:oauth2_issuer],
        developer_token:    
Rails.application.secrets.adwords[:developer_token],
        client_customer_id: customer_id
      },
      service: {
        environment: 'PRODUCTION'
      },
      connection: {
        enable_gzip: false
      },
      library: {
        log_level: 'INFO'
      }
    })
    
    @client.service(:CampaignService, :v201708).get({:fields => ['Id', 
'Name', 'Status']}

Here are the steps I did in order:

1. Log into my MCC AdWords account
2. Visit Google Developer Console
3. Create a new Project
4. Create a Service Account key. Make sure the domain checkbox is checked. 
I use the email address it generates above in `oauth2_issuer`
5. Download the JSON key
6. Get an MCC account (I actually had one I use for live campaigns, so I 
skipped this step)
7. Generate an AdWords API key from within MCC (i used it above in 
`developer_token`).
8. Create a new test manager account.
9. Inside the new test manager account, create a new test client account, 
and take that customer ID and place it above in `client_customer_id`.

I'm not sure where I went wrong, from what I can tell I followed the steps 
exactly.

Thanks

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