Google Indexing API - 403 'Forbidden Response'

I am making a call to the google indexing API for job postings:

private $client;
private $httpClient;

public function initClient($kernel)
{
    $this->client = new \Google_Client();
    $this->client->setAuthConfig(JSON_KEY_HERE);
    $this->client->addScope('https://www.googleapis.com/auth/indexing');
    $this->httpClient = $this->client->authorize();
}

public function sendJob()
{
    $endpoint = 'https://indexing.googleapis.com/v3/urlNotifications:publish';
    $content = "{
                    \"url\": \"URL_HERE\",
                    \"type\": \"URL_UPDATED\"
                }";

    $response = $this->httpClient->post($endpoint, array('body' => $content));
}

When making the call to the API, the response given is '403 - Forbidden' .

Any ideas what this error actually means? I have created the service account correctly but cannot replicate success from my dev enviroment.

Make sure that you have added the service account as an Owner in Google Search Console as described here: https://developers.google.com/search/apis/indexing-api/v3/prereqs#verify-site.

My problem was that we had multiple entries for the domain in the Search Console (with and without www and with and without https); after adding the service account as an owner to all 4 entries it's working.