Log in to the Magento server, go to your Magento install directory and run following commands:
composer require rohitkundale/order-comment
php -f bin/magento module:enable RohitKundale_OrderComment
php -f bin/magento setup:upgrade
rm -rf pub/static/*; rm -rf var/view_preprocessed/*;
php -f bin/magento setup:static-content:deploy