openssl_sign(): supplied key param cannot be coerced into a private key

I can't add a document into a collection.

$serviceAccount = ServiceAccount::fromJsonFile( storage_path()  . '/app/google-services.json');

        $firestore = (new Factory)
            ->withServiceAccount($serviceAccount)
            ->createFirestore();

        $collection = $firestore->collection('alerts');

        $doc = $collection->document('name of new document');
        $doc->set(['body' => 'est','date' => 'est','image' => 'est','headline' => 'est' ] );

The above code throws open_ssl() error :

In ApiClient.php line 58:

  openssl_sign(): supplied key param cannot be coerced into a private key  

In JWT.php line 199:

  openssl_sign(): supplied key param cannot be coerced into a private

I am using the bellow json format for google-services.json

{
  "project_id": "xxxx",
  "private_key": "xxxx",
  "client_id": "xxxx",
  "client_email": "xxx"
}

morrislaptop / firestore-php

openssl_sign(): supplied key param cannot be coerced into a private key #16