Error getting access token for service account: 401 Unauthorized\nPOST https://oauth2.googleapis.com/token; Google Calendar API

I have integrated Google Calendar with business email of my company. Everything works fine if we use email with our own domain. However, if a certain user uses with a simple gmail.com domain the error pops up that states: "Error getting access token for service account: 401 Unauthorized".

I am a newbie in Google Calendar API. Is it possible to use Calendar with business domain and a simple domain emails?

Here are my credentials:

{
  "type": "service_account",
  "project_id": "example",
  "private_key_id": "***",
  "private_key": "****",
  "client_email": "example.gserviceaccount.com",
  "client_id": "***",
  "auth_uri": "https://accounts.google.com/o/oauth2/auth",
  "token_uri": "https://oauth2.googleapis.com/token",
  "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
  "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/*****-service-account%40dms-web-281206.iam.gserviceaccount.com"
}

My code:

   static private GoogleCredentials getCredentials(String email) {
        try {
            ClassPathResource res = new ClassPathResource("credentials.json");
            InputStream inputStream = res.getInputStream();

            GoogleCredentials credentials = ServiceAccountCredentials.fromStream(inputStream).createScoped(SCOPES)
                    .createDelegated(email);
            return credentials;
        } catch (Exception e) {
            throw new ApiException(HttpStatus.INTERNAL_SERVER_ERROR.value(), ErrorCodes.GOOGLE_CLIENT_ERROR(),
                    "Google Credentials error: " + e.getMessage());
        }
    }

    private Calendar getClient(String email) {
        try {
            final NetHttpTransport HTTP_TRANSPORT = GoogleNetHttpTransport.newTrustedTransport();
            HttpRequestInitializer requestInitializer = new HttpCredentialsAdapter(getCredentials(email));

            return new Calendar.Builder(HTTP_TRANSPORT, JSON_FACTORY, requestInitializer)
                    .setApplicationName("dms-web-281206")
                    .build();
        } catch (Exception e) {
            throw new ApiException(HttpStatus.INTERNAL_SERVER_ERROR.value(), ErrorCodes.GOOGLE_CALENDAR_ERROR(),
                    "Google calendar error: " + e.getMessage());
        }
    }