putenv("GOOGLE_APPLICATION_CREDENTIALS=" . CLIENT_ACCOUNT_FILE);

$client = new Google_Client();
$client->useApplicationDefaultCredentials();
$client->setScopes(['https://www.googleapis.com/auth/youtube']);
$client->setDefer(true);

$youtube = new Google_Service_YouTube($client);
try {
  $set_request = $youtube->thumbnails->set($video_id);
  $media = new Google_Http_MediaFileUpload(
    $client,
    $set_request,
    IMAGE_MIME,
    null,
    true,
    CHUNK_SIZE_BYTES
  );

  $media->setFileSize(filesize($image_path));

  $status = false;
  $handle = fopen($image_path, "rb");
  while (!$status && !feof($handle)) {
    $chunk = fread($handle, CHUNK_SIZE_BYTES);
    $status = $media->nextChunk($chunk);
  }
  fclose($handle);
} catch (Exception $e) {
  echo "ERROR -> " . $e->getMessage();
}
$client->setDefer(false);

{
 "error": {
  "errors": [
   {
    "domain": "youtube.thumbnail",
    "reason": "forbidden",
    "message": "The thumbnail cant be set for the specified video. The request might not be properly authorized.",
    "locationType": "parameter",
    "location": "videoId"
   }
  ],
  "code": 403,
  "message": "The thumbnail cant be set for the specified video. The request might not be properly authorized."
 }
}