In Magento 2 you cannot perform the reindex from the back-office. To make a reindex you need use PuTTY or another terminal application to run the CLI commands, e.g. command for reindex:
php bin/magento indexer:reindex
With FREE Magento 2 Command Line (CLI) Extension by Magefan you can run CLI commands directly from Magento 2 admin panel.
Once you install Magefan CLI extension a new menu item "System > Command Line" will be added to the admin menu.
On "Comand Line" page you need to enter your current admin user password (for security reason) and enter the command to start reindex.
After you have entered a command "php bin/magento indexer:reindex" press ENTER to start the reindexing.
With Magefan CLI module, you can run any other Magento 2 commands.