I am desperate to resolve this task, so I hope to get the answer here.
I set up the service account in my GSuit and my goal is to get all the emails and calendars of all the users in my company.

Steps, that I have done:

Created a new API app

Enable access to APIs

Enabled API apps Google Calendar API, Gmail API, Admin SDK API, Google People API, Contacts API

OAuth consent screen. Internal. Add scopes:

/auth/admin.directory.user.readonly

mail.google.com/

/auth/profile.emails.read

/auth/gmail.readonly

/auth/user.emails.read

/auth/userinfo.email

Created Service Account. Granted permissions “Service Account Token Creator”, Viewer.

Security > API Controls > Domain-wide Delegation to my Service Account client ID:

mail.google.com,

/auth/contacts,

/auth/gmail.readonly,

/auth/userinfo.email,

auth/userinfo.profile,

auth/admin.directory.user,
auth/admin.directory.user.readonly

I can get Users but not emails.

It throws the error:

The service gmail has thrown an exception. HttpStatusCode is BadRequest.

Google.Apis.Requests.RequestError Precondition check failed.

[400] Errors [ Message[Precondition check failed.] Location[ – ] Reason[failedPrecondition] Domain[global] ]

Google.GoogleApiException: The service gmail has thrown an exception.

HttpStatusCode is BadRequest. The precondition check failed.

And my code is simple as:

string[] scopes = { DirectoryService.Scope.AdminDirectoryUser, GmailService.Scope.GmailReadonly, CalendarService.Scope.CalendarReadonly };

GoogleCredential credential;
using (var stream = new FileStream(keyFilePath, FileMode.Open, FileAccess.Read))
{
    credential = GoogleCredential.FromStream(stream)
        .CreateWithUser("my service account email")
        .CreateScoped(scopes);
}

var service = new DirectoryService(new BaseClientService.Initializer()
{
    HttpClientInitializer = credential,
    ApplicationName = "Your Application Name",
});

// Call the API to get users
var users = service.Users.List();
users.Customer = "My company's CustomerID";

var response = users.Execute();
var userList = response.UsersValue;

var serviceGmail = new GmailService(new BaseClientService.Initializer()
{
    HttpClientInitializer = credential,
    ApplicationName = "Your Application Name",
});

foreach (var user in userList)
{
    var request = serviceGmail.Users.Messages.List(user.Id);
    ListMessagesResponse emails = request.Execute();
    var messages = emails.Messages;  
}

I followed the oficial documentation and checked out tons of the stach overflow articles.