Failed to load JSON secrets file for service account configuration. System.Exception {Google.Api.Ads.Common.Lib.AdsOAuthException

Question

I am trying to call Google Ad Manager Api for the first time in C#. Afer following the tutorial at https://developers.google.com/ad-manager/api/start

This is my code:

try
{
    AdManagerUser user = new AdManagerUser();
    using (NetworkService networkService = user.GetService<NetworkService>())
    {
        Network network = networkService.getCurrentNetwork();
        Console.WriteLine($"Current network has network code {network.networkCode} and" +
                          $"display name {network.displayName}");
    }
}
catch (Exception e)
{
    Console.WriteLine(e);
    throw;
}

But it breaks at the first line with this exception:

Failed to load JSON secrets file for service account configuration. See inner exception for details."} System.Exception {Google.Api.Ads.Common.Lib.AdsOAuthException

In the App.config, this is the configuration:

  <AdManagerApi>
    <add key="MaskCredentials" value="true"/>
    <add key="EnableGzipCompression" value="true"/>
    <add key="IncludeUtilitiesInUserAgent" value="true"/>

    <!-- Proxy settings for library. -->
    <add key="ProxyServer" value=""/>
    <add key="ProxyUser" value=""/>
    <add key="ProxyPassword" value=""/>
    <add key="ProxyDomain" value=""/>

    <add key="ApplicationName" value="test_app"/>
    <add key="NetworkCode" value="xxx"/>

    <add key="AuthorizationMethod" value="OAuth2"/>
    
    <add key="OAuth2ClientId" value="xxx"/>
    <add key="OAuth2ClientSecret" value="xxx"/>
    
    <add key="OAuth2Mode" value="SERVICE_ACCOUNT" />
    <add key="OAuth2SecretsJsonPath" value="C:\Secret\xxx.json" />
    
  </AdManagerApi>