Receiving Insufficient Permission error from DirectoryService

2 options:

1. You didn't include the right Scope. Are you sure that DirectoryService.Scope.AdminDirectoryOrgunit, DirectoryService.Scope.AdminDirectoryUser are enough?
2. Did you enable the API in the Console? More information is available at: https://developers.google.com/api-client-library/dotnet/get_started#auth, Look for your project in https://console.cloud.google.com/project and make sure that you enabled the Directory Admin API.

Please update this thread if one of these options worked or something else is still missing for you.

Scopes

It appears that you are trying this Quickstart:

• .NET Quickstart for Directory API

However, the scope(s) used in that tuturoial are:

new [] { DirectoryService.Scope.AdminDirectoryUserReadonly };

However, in the code your posted code you have:

new[] { DirectoryService.Scope.AdminDirectoryOrgunit, DirectoryService.Scope.AdminDirectoryUser },

Tokens

After you change your scopes (shown above), you may have to delete your OAuth2 token, and then re-authorize access for your application. (Unless you haven't done the "authorize access" step yet.)

\token.json\Google.Apis.Auth.OAuth2.Responses.TokenResponse-user

Enable APIs

Also, as I think you already discovered, enabling the Directory API is different process than enabling the Gmail API (and found at different URLs)

Enable Directory API

Enable Gmail API