The error message suggests that the projectId parameter is missing. You need to provide the projectId when creating an instance of the Kreait\Firebase\Auth\ApiClient class.
Here's an example of how to create an instance of the Kreait\Firebase\Auth\ApiClient class with the projectId parameter:
use Kreait\Firebase\Factory;
$factory = (new Factory)
->withServiceAccount('/path/to/service-account.json')
->withDatabaseUri('https://your-database.firebaseio.com')
->withProjectId('your-project-id');
$auth = $factory->createAuth();
Make sure to replace your-project-id with your actual Firebase project ID.