If you followed all previous answears and the upgrade guide and still had no success, make sure you have every package that is referenced in your project in your composer.json, for example if you have something like: "Barryvdh\DomPDF\ServiceProvider::class" or "use Barryvdh\DomPDF\PDF" in your code, you have to make sure that in your composer.json you have "barryvdh/laravel-dompdf", or if you don't need or don't want it anymore remove it from your code, this can happen when you merge code from other branchs but your composer.json file is not in sync with other branchs, the error message isn't much helpful, but take a look at your log file storage/logs/laravel.log