Fatal error: Uncaught Error: Class 'Google_client' not found despite adding library through composer

Question

Getting error Fatal error: Uncaught Error: Class 'Google_client' not found.

Using php -v 7.1.21

Installed google api php client via composer using

composer require google/apiclient:"^2.0"

Using

require 'vendor/autoload.php';

to include library.

What am I doing wrong ?