In this post, you will find the solution for issue :.
FatalThrowableError in Client.php line 81: Class 'MongoDB\Driver\Manager' not found
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 Ubuntu :
sudo apt-get install php-mongodb
Now restart the server :
sudo service apache2 restart