Originally you cannot reindex Magento from admin panel. To perform Magento 2 reindex you need to use PuTTY or another terminal application to run the CLI commands, e.g. command for reindexing:
php bin/magento indexer:reindex
To reindex Magento 2 from admin panel:
1. Install the Magefan CLI extension to see a new menu item "System > Command Line" to be added to the admin menu.
2. Enter your current admin user password (for security reasons) On the "Comand Line" page and enter the Magento 2 reindex command.
3. Press ENTER to start the reindexing after you have entered the "php bin/magento indexer:reindex" command.
With the Magefan CLI module, you can run any other Magento 2 commands.
Check out the video to see how easy it actually is: