In this tutorial, you will find the solution for this issue FatalThrowableError in Client.php line 81: Class 'MongoDB\Driver\Manager' not found, If you also fetch this issue with mongodb using jenssegers/laravel-mongodb composer package then you can solve your issue from here.
When you are going to connect MongoDB database with any PHP application then you can face this issue if you don't have PHP MongoDB driver.
So first you need to run following command to install Mongodb PHP extension in Your Ubuntu :
sudo apt-get install php-mongodb
Now restart the server :
sudo service apache2 restart
I hope it can help you....