require 'vendor/autoload.php';
$sec = 10;
$movie = 'test.mp4';
$thumbnail = 'thumbnail.png';
$ffmpeg = FFMpeg\FFMpeg::create(); //ошибка тут
$video = $ffmpeg->open($movie);
$frame = $video->frame(FFMpeg\Coordinate\TimeCode::fromSeconds($sec));// и тут
$frame->save($thumbnail);